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Date: Wed, 01 Dec 2010 20:48:57 +0100
From: Eric Dumazet <eric.dumazet@...il.com>
To: David Miller <davem@...emloft.net>
Cc: hagen@...u.net, xiaosuo@...il.com, wirelesser@...il.com,
netdev@...r.kernel.org
Subject: Re: multi bpf filter will impact performance?
Le mercredi 01 décembre 2010 à 10:44 -0800, David Miller a écrit :
> From: Eric Dumazet <eric.dumazet@...il.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
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..1e713b3 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 +413,63 @@ 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;
+
+ 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))) {
+ pr_err("filter: bad load(%d) memvalid=%x\n", filter[pc].k, memvalid);
+ 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
@@ -432,6 +483,7 @@ EXPORT_SYMBOL(sk_run_filter);
* All jumps are forward as they are not signed.
*
* Returns 0 if the rule set is legal or -EINVAL if not.
+ *
*/
int sk_chk_filter(struct sock_filter *filter, int flen)
{
@@ -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_load_and_stores(filter, flen);
}
return -EINVAL;
}
--
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