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authorEric Dumazet <eric.dumazet@gmail.com>2010-12-01 20:46:24 +0000
committerDavid S. Miller <davem@davemloft.net>2010-12-06 12:59:09 -0800
commit2d5311e4e8272fd398fc1cf278f12fd6dee4074b (patch)
treed86fcab7044baa6ec41ef87b8eca186ae6f9ea89 /net/core
parentae9c416d686db74f67d73c1bebf1e3a7e8b3c5b5 (diff)
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filter: add a security check at install time
We added some security checks in commit 57fe93b374a6 (filter: make sure filters dont read uninitialized memory) to close a potential leak of kernel information to user. This added a potential extra cost at run time, while we can perform a check of the filter itself, to make sure a malicious user doesnt try to abuse us. This patch adds a check_loads() function, whole unique purpose is to make this check, allocating a temporary array of mask. We scan the filter and propagate a bitmask information, telling us if a load M(K) is allowed because a previous store M(K) is guaranteed. (So that sk_run_filter() can possibly not read unitialized memory) Note: this can uncover application bug, denying a filter attach, previously allowed. Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com> Cc: Dan Rosenberg <drosenberg@vsecurity.com> Cc: Changli Gao <xiaosuo@gmail.com> Acked-by: Changli Gao <xiaosuo@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net/core')
-rw-r--r--net/core/filter.c72
1 files changed, 63 insertions, 9 deletions
diff --git a/net/core/filter.c b/net/core/filter.c
index 054e286..ac4920a 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -166,11 +166,9 @@ unsigned int sk_run_filter(struct sk_buff *skb, const struct sock_filter *fentry
u32 A = 0; /* Accumulator */
u32 X = 0; /* Index Register */
u32 mem[BPF_MEMWORDS]; /* Scratch Memory Store */
- unsigned long memvalid = 0;
u32 tmp;
int k;
- BUILD_BUG_ON(BPF_MEMWORDS > BITS_PER_LONG);
/*
* Process array of filter instructions.
*/
@@ -318,12 +316,10 @@ load_b:
X = K;
continue;
case BPF_S_LD_MEM:
- A = (memvalid & (1UL << K)) ?
- mem[K] : 0;
+ A = mem[K];
continue;
case BPF_S_LDX_MEM:
- X = (memvalid & (1UL << K)) ?
- mem[K] : 0;
+ X = mem[K];
continue;
case BPF_S_MISC_TAX:
X = A;
@@ -336,11 +332,9 @@ load_b:
case BPF_S_RET_A:
return A;
case BPF_S_ST:
- memvalid |= 1UL << K;
mem[K] = A;
continue;
case BPF_S_STX:
- memvalid |= 1UL << K;
mem[K] = X;
continue;
default:
@@ -425,6 +419,66 @@ load_b:
}
EXPORT_SYMBOL(sk_run_filter);
+/*
+ * Security :
+ * A BPF program is able to use 16 cells of memory to store intermediate
+ * values (check u32 mem[BPF_MEMWORDS] in sk_run_filter())
+ * As we dont want to clear mem[] array for each packet going through
+ * sk_run_filter(), we check that filter loaded by user never try to read
+ * a cell if not previously written, and we check all branches to be sure
+ * a malicious user doesnt try to abuse us.
+ */
+static int check_load_and_stores(struct sock_filter *filter, int flen)
+{
+ u16 *masks, memvalid = 0; /* one bit per cell, 16 cells */
+ int pc, ret = 0;
+
+ BUILD_BUG_ON(BPF_MEMWORDS > 16);
+ masks = kmalloc(flen * sizeof(*masks), GFP_KERNEL);
+ if (!masks)
+ return -ENOMEM;
+ memset(masks, 0xff, flen * sizeof(*masks));
+
+ for (pc = 0; pc < flen; pc++) {
+ memvalid &= masks[pc];
+
+ switch (filter[pc].code) {
+ case BPF_S_ST:
+ case BPF_S_STX:
+ memvalid |= (1 << filter[pc].k);
+ break;
+ case BPF_S_LD_MEM:
+ case BPF_S_LDX_MEM:
+ if (!(memvalid & (1 << filter[pc].k))) {
+ ret = -EINVAL;
+ goto error;
+ }
+ break;
+ case BPF_S_JMP_JA:
+ /* a jump must set masks on target */
+ masks[pc + 1 + filter[pc].k] &= memvalid;
+ memvalid = ~0;
+ break;
+ case BPF_S_JMP_JEQ_K:
+ case BPF_S_JMP_JEQ_X:
+ case BPF_S_JMP_JGE_K:
+ case BPF_S_JMP_JGE_X:
+ case BPF_S_JMP_JGT_K:
+ case BPF_S_JMP_JGT_X:
+ case BPF_S_JMP_JSET_X:
+ case BPF_S_JMP_JSET_K:
+ /* a jump must set masks on targets */
+ masks[pc + 1 + filter[pc].jt] &= memvalid;
+ masks[pc + 1 + filter[pc].jf] &= memvalid;
+ memvalid = ~0;
+ break;
+ }
+ }
+error:
+ kfree(masks);
+ return ret;
+}
+
/**
* sk_chk_filter - verify socket filter code
* @filter: filter to verify
@@ -553,7 +607,7 @@ int sk_chk_filter(struct sock_filter *filter, int flen)
switch (filter[flen - 1].code) {
case BPF_S_RET_K:
case BPF_S_RET_A:
- return 0;
+ return check_load_and_stores(filter, flen);
}
return -EINVAL;
}