Commit 2d5311e4 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

filter: add a security check at install time 



We added some security checks in commit 57fe93b3
(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: default avatarEric Dumazet <eric.dumazet@gmail.com>
Cc: Dan Rosenberg <drosenberg@vsecurity.com>
Cc: Changli Gao <xiaosuo@gmail.com>
Acked-by: default avatarChangli Gao <xiaosuo@gmail.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent ae9c416d

net/core/filter.c

+63 −9
Original line number Diff line number Diff line
@@ -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;

}

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