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filter: add a security check at install time 

We added some security checks in commit 57fe93b374
(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>
This commit is contained in:
Eric Dumazet 2010-12-01 20:46:24 +00:00 committed by David S. Miller
parent ae9c416d68
commit 2d5311e4e8
1 changed files with 63 additions and 9 deletions
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72
net/core/filter.c
View File

@ -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;
}