[PATCH] uml: skas0 - separate kernel address space on stock hosts

UML has had two modes of operation - an insecure, slow mode (tt mode) in
which the kernel is mapped into every process address space which requires
no host kernel modifications, and a secure, faster mode (skas mode) in
which the UML kernel is in a separate host address space, which requires a
patch to the host kernel.

This patch implements something very close to skas mode for hosts which
don't support skas - I'm calling this skas0.  It provides the security of
the skas host patch, and some of the performance gains.

The two main things that are provided by the skas patch, /proc/mm and
PTRACE_FAULTINFO, are implemented in a way that require no host patch.

For the remote address space changing stuff (mmap, munmap, and mprotect),
we set aside two pages in the process above its stack, one of which
contains a little bit of code which can call mmap et al.

To update the address space, the system call information (system call
number and arguments) are written to the stub page above the code.  The
%esp is set to the beginning of the data, the %eip is set the the start of
the stub, and it repeatedly pops the information into its registers and
makes the system call until it sees a system call number of zero.  This is
to amortize the cost of the context switch across multiple address space
updates.

When the updates are done, it SIGSTOPs itself, and the kernel process
continues what it was doing.

For a PTRACE_FAULTINFO replacement, we set up a SIGSEGV handler in the
child, and let it handle segfaults rather than nullifying them.  The
handler is in the same page as the mmap stub.  The second page is used as
the stack.  The handler reads cr2 and err from the sigcontext, sticks them
at the base of the stack in a faultinfo struct, and SIGSTOPs itself.  The
kernel then reads the faultinfo and handles the fault.

A complication on x86_64 is that this involves resetting the registers to
the segfault values when the process is inside the kill system call.  This
breaks on x86_64 because %rcx will contain %rip because you tell SYSRET
where to return to by putting the value in %rcx.  So, this corrupts $rcx on
return from the segfault.  To work around this, I added an
arch_finish_segv, which on x86 does nothing, but which on x86_64 ptraces
the child back through the sigreturn.  This causes %rcx to be restored by
sigreturn and avoids the corruption.  Ultimately, I think I will replace
this with the trick of having it send itself a blocked signal which will be
unblocked by the sigreturn.  This will allow it to be stopped just after
the sigreturn, and PTRACE_SYSCALLed without all the back-and-forth of
PTRACE_SYSCALLing it through sigreturn.

This runs on a stock host, so theoretically (and hopefully), tt mode isn't
needed any more.  We need to make sure that this is better in every way
than tt mode, though.  I'm concerned about the speed of address space
updates and page fault handling, since they involve extra round-trips to
the child.  We can amortize the round-trip cost for large address space
updates by writing all of the operations to the data page and having the
child execute them all at the same time.  This will help fork and exec, but
not page faults, since they involve only one page.

I can't think of any way to help page faults, except to add something like
PTRACE_FAULTINFO to the host.  There is PTRACE_SIGINFO, but UML doesn't use
siginfo for SIGSEGV (or anything else) because there isn't enough
information in the siginfo struct to handle page faults (the faulting
operation type is missing).  Adding that would make PTRACE_SIGINFO a usable
equivalent to PTRACE_FAULTINFO.

As for the code itself:

- The system call stub is in arch/um/kernel/sys-$(SUBARCH)/stub.S.  It is
  put in its own section of the binary along with stub_segv_handler in
  arch/um/kernel/skas/process.c.  This is manipulated with run_syscall_stub
  in arch/um/kernel/skas/mem_user.c.  syscall_stub will execute any system
  call at all, but it's only used for mmap, munmap, and mprotect.

- The x86_64 stub calls sigreturn by hand rather than allowing the normal
  sigreturn to happen, because the normal sigreturn is a SA_RESTORER in
  UML's address space provided by libc.  Needless to say, this is not
  available in the child's address space.  Also, it does a couple of odd
  pops before that which restore the stack to the state it was in at the
  time the signal handler was called.

- There is a new field in the arch mmu_context, which is now a union.
  This is the pid to be manipulated rather than the /proc/mm file
  descriptor.  Code which deals with this now checks proc_mm to see whether
  it should use the usual skas code or the new code.

- userspace_tramp is now used to create a new host process for every UML
  process, rather than one per UML processor.  It checks proc_mm and
  ptrace_faultinfo to decide whether to map in the pages above its stack.

- start_userspace now makes CLONE_VM conditional on proc_mm since we need
  separate address spaces now.

- switch_mm_skas now just sets userspace_pid[0] to the new pid rather
  than PTRACE_SWITCH_MM.  There is an addition to userspace which updates
  its idea of the pid being manipulated each time around the loop.  This is
  important on exec, when the pid will change underneath userspace().

- The stub page has a pte, but it can't be mapped in using tlb_flush
  because it is part of tlb_flush.  This is why it's required for it to be
  mapped in by userspace_tramp.

Other random things:

- The stub section in uml.lds.S is page aligned.  This page is written
  out to the backing vm file in setup_physmem because it is mapped from
  there into user processes.

- There's some confusion with TASK_SIZE now that there are a couple of
  extra pages that the process can't use.  TASK_SIZE is considered by the
  elf code to be the usable process memory, which is reasonable, so it is
  decreased by two pages.  This confuses the definition of
  USER_PGDS_IN_LAST_PML4, making it too small because of the rounding down
  of the uneven division.  So we round it to the nearest PGDIR_SIZE rather
  than the lower one.

- I added a missing PT_SYSCALL_ARG6_OFFSET macro.

- um_mmu.h was made into a userspace-usable file.

- proc_mm and ptrace_faultinfo are globals which say whether the host
  supports these features.

- There is a bad interaction between the mm.nr_ptes check at the end of
  exit_mmap, stack randomization, and skas0.  exit_mmap will stop freeing
  pages at the PGDIR_SIZE boundary after the last vma.  If the stack isn't
  on the last page table page, the last pte page won't be freed, as it
  should be since the stub ptes are there, and exit_mmap will BUG because
  there is an unfreed page.  To get around this, TASK_SIZE is set to the
  next lowest PGDIR_SIZE boundary and mm->nr_ptes is decremented after the
  calls to init_stub_pte.  This ensures that we know the process stack (and
  all other process mappings) will be below the top page table page, and
  thus we know that mm->nr_ptes will be one too many, and can be
  decremented.

Things that need fixing:

- We may need better assurrences that the stub code is PIC.

- The stub pte is set up in init_new_context_skas.

- alloc_pgdir is probably the right place.

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

10 files changed, 470 insertions, 152 deletions

diff --git a/arch/um/kernel/skas/exec_kern.c b/arch/um/kernel/skas/exec_kern.c
index c6b4d5d..77ed7bb 100644
--- a/arch/um/kernel/skas/exec_kern.c
+++ b/arch/um/kernel/skas/exec_kern.c
@@ -18,7 +18,7 @@
 void flush_thread_skas(void)
 {
 	force_flush_all();
-	switch_mm_skas(current->mm->context.skas.mm_fd);
+        switch_mm_skas(&current->mm->context.skas.id);
 }
 
 void start_thread_skas(struct pt_regs *regs, unsigned long eip, 
diff --git a/arch/um/kernel/skas/include/mm_id.h b/arch/um/kernel/skas/include/mm_id.h
new file mode 100644
index 0000000..48dd098
--- /dev/null
+++ b/arch/um/kernel/skas/include/mm_id.h
@@ -0,0 +1,17 @@
+/*
+ * Copyright (C) 2005 Jeff Dike (jdike@karaya.com)
+ * Licensed under the GPL
+ */
+
+#ifndef __MM_ID_H
+#define __MM_ID_H
+
+struct mm_id {
+	union {
+		int mm_fd;
+		int pid;
+	} u;
+	unsigned long stack;
+};
+
+#endif
diff --git a/arch/um/kernel/skas/include/mmu-skas.h b/arch/um/kernel/skas/include/mmu-skas.h
index 4cd60d7..278b72f 100644
--- a/arch/um/kernel/skas/include/mmu-skas.h
+++ b/arch/um/kernel/skas/include/mmu-skas.h
@@ -6,10 +6,15 @@
 #ifndef __SKAS_MMU_H
 #define __SKAS_MMU_H
 
+#include "mm_id.h"
+
 struct mmu_context_skas {
-	int mm_fd;
+	struct mm_id id;
+        unsigned long last_page_table;
 };
 
+extern void switch_mm_skas(struct mm_id * mm_idp);
+
 #endif
 
 /*
diff --git a/arch/um/kernel/skas/include/skas.h b/arch/um/kernel/skas/include/skas.h
index 96b51db..d91a60f 100644
--- a/arch/um/kernel/skas/include/skas.h
+++ b/arch/um/kernel/skas/include/skas.h
@@ -6,9 +6,11 @@
 #ifndef __SKAS_H
 #define __SKAS_H
 
+#include "mm_id.h"
 #include "sysdep/ptrace.h"
 
 extern int userspace_pid[];
+extern int proc_mm, ptrace_faultinfo;
 
 extern void switch_threads(void *me, void *next);
 extern void thread_wait(void *sw, void *fb);
@@ -22,16 +24,17 @@ extern void new_thread_proc(void *stack, void (*handler)(int sig));
 extern void remove_sigstack(void);
 extern void new_thread_handler(int sig);
 extern void handle_syscall(union uml_pt_regs *regs);
-extern void map(int fd, unsigned long virt, unsigned long len, int r, int w,
-		int x, int phys_fd, unsigned long long offset);
-extern int unmap(int fd, void *addr, unsigned long len);
-extern int protect(int fd, unsigned long addr, unsigned long len, 
-		   int r, int w, int x);
+extern int map(struct mm_id * mm_idp, unsigned long virt, unsigned long len,
+               int r, int w, int x, int phys_fd, unsigned long long offset);
+extern int unmap(struct mm_id * mm_idp, void *addr, unsigned long len);
+extern int protect(struct mm_id * mm_idp, unsigned long addr,
+		   unsigned long len, int r, int w, int x);
 extern void user_signal(int sig, union uml_pt_regs *regs, int pid);
 extern int new_mm(int from);
-extern void start_userspace(int cpu);
+extern int start_userspace(unsigned long stub_stack);
 extern void get_skas_faultinfo(int pid, struct faultinfo * fi);
 extern long execute_syscall_skas(void *r);
+extern unsigned long current_stub_stack(void);
 
 #endif
 
diff --git a/arch/um/kernel/skas/mem.c b/arch/um/kernel/skas/mem.c
index 438db2f..147466d 100644
--- a/arch/um/kernel/skas/mem.c
+++ b/arch/um/kernel/skas/mem.c
@@ -5,7 +5,9 @@
 
 #include "linux/config.h"
 #include "linux/mm.h"
+#include "asm/pgtable.h"
 #include "mem_user.h"
+#include "skas.h"
 
 unsigned long set_task_sizes_skas(int arg, unsigned long *host_size_out, 
 				  unsigned long *task_size_out)
@@ -18,7 +20,9 @@ unsigned long set_task_sizes_skas(int arg, unsigned long *host_size_out,
 	*task_size_out = CONFIG_HOST_TASK_SIZE;
 #else
 	*host_size_out = top;
-	*task_size_out = top;
+	if (proc_mm && ptrace_faultinfo)
+		*task_size_out = top;
+	else *task_size_out = CONFIG_STUB_START & PGDIR_MASK;
 #endif
 	return(((unsigned long) set_task_sizes_skas) & ~0xffffff);
 }
diff --git a/arch/um/kernel/skas/mem_user.c b/arch/um/kernel/skas/mem_user.c
index 1310bf1..b0980ff 100644
--- a/arch/um/kernel/skas/mem_user.c
+++ b/arch/um/kernel/skas/mem_user.c
@@ -3,100 +3,171 @@
  * Licensed under the GPL
  */
 
+#include <signal.h>
 #include <errno.h>
 #include <sys/mman.h>
+#include <sys/wait.h>
+#include <asm/page.h>
+#include <asm/unistd.h>
 #include "mem_user.h"
 #include "mem.h"
+#include "mm_id.h"
 #include "user.h"
 #include "os.h"
 #include "proc_mm.h"
-
-void map(int fd, unsigned long virt, unsigned long len, int r, int w,
-	 int x, int phys_fd, unsigned long long offset)
+#include "ptrace_user.h"
+#include "user_util.h"
+#include "kern_util.h"
+#include "task.h"
+#include "registers.h"
+#include "uml-config.h"
+#include "sysdep/ptrace.h"
+#include "sysdep/stub.h"
+#include "skas.h"
+
+extern unsigned long syscall_stub, __syscall_stub_start;
+
+extern void wait_stub_done(int pid, int sig, char * fname);
+
+static long run_syscall_stub(struct mm_id * mm_idp, int syscall,
+                             unsigned long *args)
 {
-	struct proc_mm_op map;
-	int prot, n;
-
-	prot = (r ? PROT_READ : 0) | (w ? PROT_WRITE : 0) | 
-		(x ? PROT_EXEC : 0);
-
-	map = ((struct proc_mm_op) { .op 	= MM_MMAP,
-				     .u 	= 
-				     { .mmap	= 
-				       { .addr 		= virt,
-					 .len		= len,
-					 .prot		= prot,
-					 .flags		= MAP_SHARED | 
-					                  MAP_FIXED,
-					 .fd		= phys_fd,
-					 .offset	= offset
-				       } } } );
-	n = os_write_file(fd, &map, sizeof(map));
-	if(n != sizeof(map)) 
-		printk("map : /proc/mm map failed, err = %d\n", -n);
+        int n, pid = mm_idp->u.pid;
+        unsigned long regs[MAX_REG_NR];
+
+        get_safe_registers(regs);
+        regs[REGS_IP_INDEX] = UML_CONFIG_STUB_CODE +
+                ((unsigned long) &syscall_stub -
+                 (unsigned long) &__syscall_stub_start);
+        /* XXX Don't have a define for starting a syscall */
+        regs[REGS_SYSCALL_NR] = syscall;
+        regs[REGS_SYSCALL_ARG1] = args[0];
+        regs[REGS_SYSCALL_ARG2] = args[1];
+        regs[REGS_SYSCALL_ARG3] = args[2];
+        regs[REGS_SYSCALL_ARG4] = args[3];
+        regs[REGS_SYSCALL_ARG5] = args[4];
+        regs[REGS_SYSCALL_ARG6] = args[5];
+        n = ptrace_setregs(pid, regs);
+        if(n < 0){
+                printk("run_syscall_stub : PTRACE_SETREGS failed, "
+                       "errno = %d\n", n);
+                return(n);
+        }
+
+        wait_stub_done(pid, 0, "run_syscall_stub");
+
+        return(*((unsigned long *) mm_idp->stack));
 }
 
-int unmap(int fd, void *addr, unsigned long len)
+int map(struct mm_id *mm_idp, unsigned long virt, unsigned long len,
+        int r, int w, int x, int phys_fd, unsigned long long offset)
 {
-	struct proc_mm_op unmap;
-	int n;
-
-	unmap = ((struct proc_mm_op) { .op 	= MM_MUNMAP,
-				       .u 	= 
-				       { .munmap	= 
-					 { .addr 	= (unsigned long) addr,
-					   .len		= len } } } );
-	n = os_write_file(fd, &unmap, sizeof(unmap));
-	if(n != sizeof(unmap)) {
-		if(n < 0)
-			return(n);
-		else if(n > 0)
-			return(-EIO);
-	}
-
-	return(0);
+        int prot, n;
+
+        prot = (r ? PROT_READ : 0) | (w ? PROT_WRITE : 0) |
+                (x ? PROT_EXEC : 0);
+
+        if(proc_mm){
+                struct proc_mm_op map;
+                int fd = mm_idp->u.mm_fd;
+                map = ((struct proc_mm_op) { .op	= MM_MMAP,
+                                             .u		=
+                                             { .mmap	=
+                                               { .addr	= virt,
+                                                 .len	= len,
+                                                 .prot	= prot,
+                                                 .flags	= MAP_SHARED |
+                                                 MAP_FIXED,
+                                                 .fd	= phys_fd,
+                                                 .offset= offset
+                                               } } } );
+                n = os_write_file(fd, &map, sizeof(map));
+                if(n != sizeof(map))
+                        printk("map : /proc/mm map failed, err = %d\n", -n);
+        }
+        else {
+                long res;
+                unsigned long args[] = { virt, len, prot,
+                                         MAP_SHARED | MAP_FIXED, phys_fd,
+                                         MMAP_OFFSET(offset) };
+
+                res = run_syscall_stub(mm_idp, STUB_MMAP_NR, args);
+                if((void *) res == MAP_FAILED)
+                        printk("mmap stub failed, errno = %d\n", res);
+        }
+
+        return 0;
 }
 
-int protect(int fd, unsigned long addr, unsigned long len, int r, int w, 
-	    int x, int must_succeed)
+int unmap(struct mm_id *mm_idp, void *addr, unsigned long len)
 {
-	struct proc_mm_op protect;
-	int prot, n;
-
-	prot = (r ? PROT_READ : 0) | (w ? PROT_WRITE : 0) | 
-		(x ? PROT_EXEC : 0);
-
-	protect = ((struct proc_mm_op) { .op 	= MM_MPROTECT,
-				       .u 	= 
-				       { .mprotect	= 
-					 { .addr 	= (unsigned long) addr,
-					   .len		= len,
-					   .prot	= prot } } } );
-
-	n = os_write_file(fd, &protect, sizeof(protect));
-	if(n != sizeof(protect)) {
-		if(n == 0) return(0);
-
-		if(must_succeed)
-			panic("protect failed, err = %d", -n);
-
-		return(-EIO);
-	}
+        int n;
+
+        if(proc_mm){
+                struct proc_mm_op unmap;
+                int fd = mm_idp->u.mm_fd;
+                unmap = ((struct proc_mm_op) { .op	= MM_MUNMAP,
+                                               .u	=
+                                               { .munmap	=
+                                                 { .addr	=
+                                                   (unsigned long) addr,
+                                                   .len		= len } } } );
+                n = os_write_file(fd, &unmap, sizeof(unmap));
+                if(n != sizeof(unmap)) {
+                        if(n < 0)
+                                return(n);
+                        else if(n > 0)
+                                return(-EIO);
+                }
+        }
+        else {
+                int res;
+                unsigned long args[] = { (unsigned long) addr, len, 0, 0, 0,
+                                         0 };
+
+                res = run_syscall_stub(mm_idp, __NR_munmap, args);
+                if(res < 0)
+                        printk("munmap stub failed, errno = %d\n", res);
+        }
+
+        return(0);
+}
 
-	return(0);
+int protect(struct mm_id *mm_idp, unsigned long addr, unsigned long len,
+	    int r, int w, int x)
+{
+        struct proc_mm_op protect;
+        int prot, n;
+
+        prot = (r ? PROT_READ : 0) | (w ? PROT_WRITE : 0) |
+                (x ? PROT_EXEC : 0);
+
+        if(proc_mm){
+                int fd = mm_idp->u.mm_fd;
+                protect = ((struct proc_mm_op) { .op	= MM_MPROTECT,
+                                                 .u	=
+                                                 { .mprotect	=
+                                                   { .addr	=
+                                                     (unsigned long) addr,
+                                                     .len	= len,
+                                                     .prot	= prot } } } );
+
+                n = os_write_file(fd, &protect, sizeof(protect));
+                if(n != sizeof(protect))
+                        panic("protect failed, err = %d", -n);
+        }
+        else {
+                int res;
+                unsigned long args[] = { addr, len, prot, 0, 0, 0 };
+
+                res = run_syscall_stub(mm_idp, __NR_mprotect, args);
+                if(res < 0)
+                        panic("mprotect stub failed, errno = %d\n", res);
+        }
+
+        return(0);
 }
 
 void before_mem_skas(unsigned long unused)
 {
 }
-
-/*
- * Overrides for Emacs so that we follow Linus's tabbing style.
- * Emacs will notice this stuff at the end of the file and automatically
- * adjust the settings for this buffer only.  This must remain at the end
- * of the file.
- * ---------------------------------------------------------------------------
- * Local variables:
- * c-file-style: "linux"
- * End:
- */
diff --git a/arch/um/kernel/skas/mmu.c b/arch/um/kernel/skas/mmu.c
index 6cb9a6d..511a855 100644
--- a/arch/um/kernel/skas/mmu.c
+++ b/arch/um/kernel/skas/mmu.c
@@ -3,46 +3,138 @@
  * Licensed under the GPL
  */
 
+#include "linux/config.h"
 #include "linux/sched.h"
 #include "linux/list.h"
 #include "linux/spinlock.h"
 #include "linux/slab.h"
+#include "linux/errno.h"
+#include "linux/mm.h"
 #include "asm/current.h"
 #include "asm/segment.h"
 #include "asm/mmu.h"
+#include "asm/pgalloc.h"
+#include "asm/pgtable.h"
 #include "os.h"
 #include "skas.h"
 
+extern int __syscall_stub_start;
+
+static int init_stub_pte(struct mm_struct *mm, unsigned long proc,
+			 unsigned long kernel)
+{
+	pgd_t *pgd;
+	pud_t *pud;
+	pmd_t *pmd;
+	pte_t *pte;
+
+	spin_lock(&mm->page_table_lock);
+	pgd = pgd_offset(mm, proc);
+	pud = pud_alloc(mm, pgd, proc);
+	if (!pud)
+		goto out;
+
+	pmd = pmd_alloc(mm, pud, proc);
+	if (!pmd)
+		goto out_pmd;
+
+	pte = pte_alloc_map(mm, pmd, proc);
+	if (!pte)
+		goto out_pte;
+
+	/* There's an interaction between the skas0 stub pages, stack
+	 * randomization, and the BUG at the end of exit_mmap.  exit_mmap
+         * checks that the number of page tables freed is the same as had
+         * been allocated.  If the stack is on the last page table page,
+	 * then the stack pte page will be freed, and if not, it won't.  To
+	 * avoid having to know where the stack is, or if the process mapped
+	 * something at the top of its address space for some other reason,
+	 * we set TASK_SIZE to end at the start of the last page table.
+	 * This keeps exit_mmap off the last page, but introduces a leak
+	 * of that page.  So, we hang onto it here and free it in
+	 * destroy_context_skas.
+	 */
+
+        mm->context.skas.last_page_table = pmd_page_kernel(*pmd);
+
+	*pte = mk_pte(virt_to_page(kernel), __pgprot(_PAGE_PRESENT));
+	*pte = pte_mkexec(*pte);
+	*pte = pte_wrprotect(*pte);
+	spin_unlock(&mm->page_table_lock);
+	return(0);
+
+ out_pmd:
+	pud_free(pud);
+ out_pte:
+	pmd_free(pmd);
+ out:
+	spin_unlock(&mm->page_table_lock);
+	return(-ENOMEM);
+}
+
 int init_new_context_skas(struct task_struct *task, struct mm_struct *mm)
 {
-	int from;
+	struct mm_struct *cur_mm = current->mm;
+	struct mm_id *mm_id = &mm->context.skas.id;
+	unsigned long stack;
+	int from, ret;
 
-	if((current->mm != NULL) && (current->mm != &init_mm))
-		from = current->mm->context.skas.mm_fd;
-	else from = -1;
+	if(proc_mm){
+		if((cur_mm != NULL) && (cur_mm != &init_mm))
+			from = cur_mm->context.skas.id.u.mm_fd;
+		else from = -1;
 
-	mm->context.skas.mm_fd = new_mm(from);
-	if(mm->context.skas.mm_fd < 0){
-		printk("init_new_context_skas - new_mm failed, errno = %d\n",
-		       mm->context.skas.mm_fd);
-		return(mm->context.skas.mm_fd);
+		ret = new_mm(from);
+		if(ret < 0){
+			printk("init_new_context_skas - new_mm failed, "
+			       "errno = %d\n", ret);
+			return ret;
+		}
+		mm_id->u.mm_fd = ret;
 	}
+	else {
+		/* This zeros the entry that pgd_alloc didn't, needed since
+		 * we are about to reinitialize it, and want mm.nr_ptes to
+		 * be accurate.
+		 */
+		mm->pgd[USER_PTRS_PER_PGD] = __pgd(0);
 
-	return(0);
+		ret = init_stub_pte(mm, CONFIG_STUB_CODE,
+				    (unsigned long) &__syscall_stub_start);
+		if(ret)
+			goto out;
+
+		ret = -ENOMEM;
+		stack = get_zeroed_page(GFP_KERNEL);
+		if(stack == 0)
+			goto out;
+		mm_id->stack = stack;
+
+		ret = init_stub_pte(mm, CONFIG_STUB_DATA, stack);
+		if(ret)
+			goto out_free;
+
+		mm->nr_ptes--;
+		mm_id->u.pid = start_userspace(stack);
+	}
+
+	return 0;
+
+ out_free:
+	free_page(mm_id->stack);
+ out:
+	return ret;
 }
 
 void destroy_context_skas(struct mm_struct *mm)
 {
-	os_close_file(mm->context.skas.mm_fd);
-}
+	struct mmu_context_skas *mmu = &mm->context.skas;
 
-/*
- * Overrides for Emacs so that we follow Linus's tabbing style.
- * Emacs will notice this stuff at the end of the file and automatically
- * adjust the settings for this buffer only.  This must remain at the end
- * of the file.
- * ---------------------------------------------------------------------------
- * Local variables:
- * c-file-style: "linux"
- * End:
- */
+	if(proc_mm)
+		os_close_file(mmu->id.u.mm_fd);
+	else {
+		os_kill_ptraced_process(mmu->id.u.pid, 1);
+		free_page(mmu->id.stack);
+		free_page(mmu->last_page_table);
+	}
+}
diff --git a/arch/um/kernel/skas/process.c b/arch/um/kernel/skas/process.c
index 773cd2b..1647abb 100644
--- a/arch/um/kernel/skas/process.c
+++ b/arch/um/kernel/skas/process.c
@@ -1,5 +1,5 @@
 /* 
- * Copyright (C) 2002 Jeff Dike (jdike@karaya.com)
+ * Copyright (C) 2002- 2004 Jeff Dike (jdike@addtoit.com)
  * Licensed under the GPL
  */
 
@@ -14,6 +14,7 @@
 #include <sys/mman.h>
 #include <sys/user.h>
 #include <asm/unistd.h>
+#include <asm/types.h>
 #include "user.h"
 #include "ptrace_user.h"
 #include "time_user.h"
@@ -21,13 +22,17 @@
 #include "user_util.h"
 #include "kern_util.h"
 #include "skas.h"
+#include "mm_id.h"
 #include "sysdep/sigcontext.h"
+#include "sysdep/stub.h"
 #include "os.h"
 #include "proc_mm.h"
 #include "skas_ptrace.h"
 #include "chan_user.h"
 #include "signal_user.h"
 #include "registers.h"
+#include "mem.h"
+#include "uml-config.h"
 #include "process.h"
 
 int is_skas_winch(int pid, int fd, void *data)
@@ -39,20 +44,55 @@ int is_skas_winch(int pid, int fd, void *data)
 	return(1);
 }
 
-void get_skas_faultinfo(int pid, struct faultinfo * fi)
+void wait_stub_done(int pid, int sig, char * fname)
 {
-	int err;
-
-        err = ptrace(PTRACE_FAULTINFO, pid, 0, fi);
-	if(err)
-                panic("get_skas_faultinfo - PTRACE_FAULTINFO failed, "
-                      "errno = %d\n", errno);
+        int n, status, err;
+
+        do {
+                if ( sig != -1 ) {
+                        err = ptrace(PTRACE_CONT, pid, 0, sig);
+                        if(err)
+                                panic("%s : continue failed, errno = %d\n",
+                                      fname, errno);
+                }
+                sig = 0;
+
+                CATCH_EINTR(n = waitpid(pid, &status, WUNTRACED));
+        } while((n >= 0) && WIFSTOPPED(status) &&
+                (WSTOPSIG(status) == SIGVTALRM));
+
+        if((n < 0) || !WIFSTOPPED(status) ||
+           (WSTOPSIG(status) != SIGUSR1 && WSTOPSIG(status != SIGTRAP))){
+                panic("%s : failed to wait for SIGUSR1/SIGTRAP, "
+                      "pid = %d, n = %d, errno = %d, status = 0x%x\n",
+                      fname, pid, n, errno, status);
+        }
+}
 
-        /* Special handling for i386, which has different structs */
-        if (sizeof(struct ptrace_faultinfo) < sizeof(struct faultinfo))
-                memset((char *)fi + sizeof(struct ptrace_faultinfo), 0,
-                       sizeof(struct faultinfo) -
-                       sizeof(struct ptrace_faultinfo));
+void get_skas_faultinfo(int pid, struct faultinfo * fi)
+{
+        int err;
+
+        if(ptrace_faultinfo){
+                err = ptrace(PTRACE_FAULTINFO, pid, 0, fi);
+                if(err)
+                        panic("get_skas_faultinfo - PTRACE_FAULTINFO failed, "
+                              "errno = %d\n", errno);
+
+                /* Special handling for i386, which has different structs */
+                if (sizeof(struct ptrace_faultinfo) < sizeof(struct faultinfo))
+                        memset((char *)fi + sizeof(struct ptrace_faultinfo), 0,
+                               sizeof(struct faultinfo) -
+                               sizeof(struct ptrace_faultinfo));
+        }
+        else {
+                wait_stub_done(pid, SIGSEGV, "get_skas_faultinfo");
+
+                /* faultinfo is prepared by the stub-segv-handler at start of
+                 * the stub stack page. We just have to copy it.
+                 */
+                memcpy(fi, (void *)current_stub_stack(), sizeof(*fi));
+        }
 }
 
 static void handle_segv(int pid, union uml_pt_regs * regs)
@@ -91,11 +131,56 @@ static void handle_trap(int pid, union uml_pt_regs *regs, int local_using_sysemu
 	handle_syscall(regs);
 }
 
-static int userspace_tramp(void *arg)
+extern int __syscall_stub_start;
+
+static int userspace_tramp(void *stack)
 {
-	init_new_thread_signals(0);
-	enable_timer();
+	void *addr;
+
 	ptrace(PTRACE_TRACEME, 0, 0, 0);
+
+	init_new_thread_signals(1);
+	enable_timer();
+
+	if(!proc_mm){
+		/* This has a pte, but it can't be mapped in with the usual
+		 * tlb_flush mechanism because this is part of that mechanism
+		 */
+		int fd;
+		__u64 offset;
+
+		fd = phys_mapping(to_phys(&__syscall_stub_start), &offset);
+		addr = mmap64((void *) UML_CONFIG_STUB_CODE, page_size(),
+			      PROT_EXEC, MAP_FIXED | MAP_PRIVATE, fd, offset);
+		if(addr == MAP_FAILED){
+			printk("mapping mmap stub failed, errno = %d\n",
+			       errno);
+			exit(1);
+		}
+
+		if(stack != NULL){
+			fd = phys_mapping(to_phys(stack), &offset);
+			addr = mmap((void *) UML_CONFIG_STUB_DATA, page_size(),
+				    PROT_READ | PROT_WRITE,
+				    MAP_FIXED | MAP_SHARED, fd, offset);
+			if(addr == MAP_FAILED){
+				printk("mapping segfault stack failed, "
+				       "errno = %d\n", errno);
+				exit(1);
+			}
+		}
+	}
+	if(!ptrace_faultinfo && (stack != NULL)){
+		unsigned long v = UML_CONFIG_STUB_CODE +
+				  (unsigned long) stub_segv_handler -
+				  (unsigned long) &__syscall_stub_start;
+
+		set_sigstack((void *) UML_CONFIG_STUB_DATA, page_size());
+		set_handler(SIGSEGV, (void *) v, SA_ONSTACK,
+			    SIGIO, SIGWINCH, SIGALRM, SIGVTALRM,
+			    SIGUSR1, -1);
+	}
+
 	os_stop_process(os_getpid());
 	return(0);
 }
@@ -105,11 +190,11 @@ static int userspace_tramp(void *arg)
 #define NR_CPUS 1
 int userspace_pid[NR_CPUS];
 
-void start_userspace(int cpu)
+int start_userspace(unsigned long stub_stack)
 {
 	void *stack;
 	unsigned long sp;
-	int pid, status, n;
+	int pid, status, n, flags;
 
 	stack = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE | PROT_EXEC,
 		     MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
@@ -117,8 +202,9 @@ void start_userspace(int cpu)
 		panic("start_userspace : mmap failed, errno = %d", errno);
 	sp = (unsigned long) stack + PAGE_SIZE - sizeof(void *);
 
-	pid = clone(userspace_tramp, (void *) sp, 
-		    CLONE_FILES | CLONE_VM | SIGCHLD, NULL);
+	flags = CLONE_FILES | SIGCHLD;
+	if(proc_mm) flags |= CLONE_VM;
+	pid = clone(userspace_tramp, (void *) sp, flags, (void *) stub_stack);
 	if(pid < 0)
 		panic("start_userspace : clone failed, errno = %d", errno);
 
@@ -140,7 +226,7 @@ void start_userspace(int cpu)
 	if(munmap(stack, PAGE_SIZE) < 0)
 		panic("start_userspace : munmap failed, errno = %d\n", errno);
 
-	userspace_pid[cpu] = pid;
+	return(pid);
 }
 
 void userspace(union uml_pt_regs *regs)
@@ -174,7 +260,9 @@ void userspace(union uml_pt_regs *regs)
 		if(WIFSTOPPED(status)){
 		  	switch(WSTOPSIG(status)){
 			case SIGSEGV:
-                                handle_segv(pid, regs);
+                                if(PTRACE_FULL_FAULTINFO || !ptrace_faultinfo)
+                                        user_signal(SIGSEGV, regs, pid);
+                                else handle_segv(pid, regs);
 				break;
 			case SIGTRAP + 0x80:
 			        handle_trap(pid, regs, local_using_sysemu);
@@ -194,6 +282,7 @@ void userspace(union uml_pt_regs *regs)
 			        printk("userspace - child stopped with signal "
 				       "%d\n", WSTOPSIG(status));
 			}
+			pid = userspace_pid[0];
 			interrupt_end();
 
 			/* Avoid -ERESTARTSYS handling in host */
@@ -334,21 +423,19 @@ void reboot_skas(void)
 	siglongjmp(initial_jmpbuf, INIT_JMP_REBOOT);
 }
 
-void switch_mm_skas(int mm_fd)
+void switch_mm_skas(struct mm_id *mm_idp)
 {
 	int err;
 
 #warning need cpu pid in switch_mm_skas
-	err = ptrace(PTRACE_SWITCH_MM, userspace_pid[0], 0, mm_fd);
-	if(err)
-		panic("switch_mm_skas - PTRACE_SWITCH_MM failed, errno = %d\n",
-		      errno);
-}
-
-void kill_off_processes_skas(void)
-{
-#warning need to loop over userspace_pids in kill_off_processes_skas
-	os_kill_ptraced_process(userspace_pid[0], 1);
+	if(proc_mm){
+		err = ptrace(PTRACE_SWITCH_MM, userspace_pid[0], 0,
+			     mm_idp->u.mm_fd);
+		if(err)
+			panic("switch_mm_skas - PTRACE_SWITCH_MM failed, "
+			      "errno = %d\n", errno);
+	}
+	else userspace_pid[0] = mm_idp->u.pid;
 }
 
 /*
diff --git a/arch/um/kernel/skas/process_kern.c b/arch/um/kernel/skas/process_kern.c
index 0a7b8aa..cbabab1 100644
--- a/arch/um/kernel/skas/process_kern.c
+++ b/arch/um/kernel/skas/process_kern.c
@@ -175,9 +175,12 @@ static int start_kernel_proc(void *unused)
 	return(0);
 }
 
+extern int userspace_pid[];
+
 int start_uml_skas(void)
 {
-	start_userspace(0);
+	if(proc_mm)
+		userspace_pid[0] = start_userspace(0);
 
 	init_new_thread_signals(1);
 
@@ -199,3 +202,31 @@ int thread_pid_skas(struct task_struct *task)
 #warning Need to look up userspace_pid by cpu
 	return(userspace_pid[0]);
 }
+
+void kill_off_processes_skas(void)
+{
+	if(proc_mm)
+#warning need to loop over userspace_pids in kill_off_processes_skas
+		os_kill_ptraced_process(userspace_pid[0], 1);
+	else {
+		struct task_struct *p;
+		int pid, me;
+
+		me = os_getpid();
+		for_each_process(p){
+			if(p->mm == NULL)
+				continue;
+
+			pid = p->mm->context.skas.id.u.pid;
+			os_kill_ptraced_process(pid, 1);
+		}
+	}
+}
+
+unsigned long current_stub_stack(void)
+{
+	if(current->mm == NULL)
+		return(0);
+
+	return(current->mm->context.skas.id.stack);
+}
diff --git a/arch/um/kernel/skas/tlb.c b/arch/um/kernel/skas/tlb.c
index 18f9a77..6230999 100644
--- a/arch/um/kernel/skas/tlb.c
+++ b/arch/um/kernel/skas/tlb.c
@@ -6,6 +6,7 @@
 
 #include "linux/stddef.h"
 #include "linux/sched.h"
+#include "linux/config.h"
 #include "linux/mm.h"
 #include "asm/page.h"
 #include "asm/pgtable.h"
@@ -17,7 +18,7 @@
 #include "os.h"
 #include "tlb.h"
 
-static void do_ops(int fd, struct host_vm_op *ops, int last)
+static void do_ops(union mm_context *mmu, struct host_vm_op *ops, int last)
 {
 	struct host_vm_op *op;
 	int i;
@@ -26,18 +27,18 @@ static void do_ops(int fd, struct host_vm_op *ops, int last)
 		op = &ops[i];
 		switch(op->type){
 		case MMAP:
-			map(fd, op->u.mmap.addr, op->u.mmap.len,
+                        map(&mmu->skas.id, op->u.mmap.addr, op->u.mmap.len,
 			    op->u.mmap.r, op->u.mmap.w, op->u.mmap.x,
 			    op->u.mmap.fd, op->u.mmap.offset);
 			break;
 		case MUNMAP:
-			unmap(fd, (void *) op->u.munmap.addr,
+                        unmap(&mmu->skas.id, (void *) op->u.munmap.addr,
 			      op->u.munmap.len);
 			break;
 		case MPROTECT:
-			protect(fd, op->u.mprotect.addr, op->u.mprotect.len,
-				op->u.mprotect.r, op->u.mprotect.w,
-				op->u.mprotect.x);
+                        protect(&mmu->skas.id, op->u.mprotect.addr,
+                                op->u.mprotect.len, op->u.mprotect.r,
+                                op->u.mprotect.w, op->u.mprotect.x);
 			break;
 		default:
 			printk("Unknown op type %d in do_ops\n", op->type);
@@ -46,12 +47,15 @@ static void do_ops(int fd, struct host_vm_op *ops, int last)
 	}
 }
 
+extern int proc_mm;
+
 static void fix_range(struct mm_struct *mm, unsigned long start_addr,
 		      unsigned long end_addr, int force)
 {
-        int fd = mm->context.skas.mm_fd;
+        if(!proc_mm && (end_addr > CONFIG_STUB_START))
+                end_addr = CONFIG_STUB_START;
 
-        fix_range_common(mm, start_addr, end_addr, force, fd, do_ops);
+        fix_range_common(mm, start_addr, end_addr, force, do_ops);
 }
 
 void __flush_tlb_one_skas(unsigned long addr)
@@ -69,16 +73,20 @@ void flush_tlb_range_skas(struct vm_area_struct *vma, unsigned long start,
 
 void flush_tlb_mm_skas(struct mm_struct *mm)
 {
+	unsigned long end;
+
 	/* Don't bother flushing if this address space is about to be
          * destroyed.
          */
         if(atomic_read(&mm->mm_users) == 0)
                 return;
 
-        fix_range(mm, 0, host_task_size, 0);
+	end = proc_mm ? task_size : CONFIG_STUB_START;
+        fix_range(mm, 0, end, 0);
 }
 
 void force_flush_all_skas(void)
 {
-        fix_range(current->mm, 0, host_task_size, 1);
+	unsigned long end = proc_mm ? task_size : CONFIG_STUB_START;
+        fix_range(current->mm, 0, end, 1);
 }