[PATCH net-next-2.6] filter: add a security check at install time

Eric Dumazet
2010-12-01T20:45:49Z

Le mercredi 01 décembre 2010 à 12:23 -0800, David Miller a écrit :
> From: Eric Dumazet <eric.dumazet@gmail.com>
> Date: Wed, 01 Dec 2010 20:48:57 +0100
>
> > Le mercredi 01 décembre 2010 à 10:44 -0800, David Miller a écrit :
> >> From: Eric Dumazet <eric.dumazet@gmail.com>
> >> Date: Wed, 01 Dec 2010 19:24:53 +0100
> >>
> >> > A third work in progress (from my side) is to add a check in
> >> > sk_chk_filter() to remove the memvalid we added lately to protect the
> >> > LOAD M(K).
> >>
> >> I understand your idea, but the static checkers are still going to
> >> complain. So better add a huge comment in sk_run_filter() explaining
> >> why the checker's complaint should be ignored :-)
> >
> > Sure, here is the patch I plan to test ASAP
>
> Looks good to me.

Yes, it survives tests I did.

I submit the patch and Cc Dan Rosenberg, I would like him to double
check it if he likes.

Thanks

[PATCH net-next-2.6] 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>
---
net/core/filter.c | 70 ++++++++++++++++++++++++++++++++++++++------
1 file changed, 61 insertions(+), 9 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index a44d27f..00a0d50 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:
@@ -419,6 +413,64 @@ 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_loads(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;
+ 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;
+ break;
+ }
+ }
+error:
+ kfree(masks);
+ return ret;
+}
+
/**
* sk_chk_filter - verify socket filter code
* @filter: filter to verify
@@ -547,7 +599,7 @@ 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_loads(filter, flen);
}
return -EINVAL;
}