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

[Eric Dumazet <eric.dumazet@gmail.com>]

Le jeudi 02 décembre 2010 à 16:11 +0800, Changli Gao a écrit :

> It seems correct to me now.
> 
> Acked-by: Changli Gao <xiaosuo@gmail.com>
> 

Thanks for reviewing Changli.

Now I am thinking about not denying the filter installation, but change
the problematic LOAD M(1)  and LOADX M(1)  by LOADI #0 (BPF_S_LD_IMM
K=0) and LOADIX #0 (BPF_S_LDX_IMM K=0)

(ie pretend the value of memory is 0, not a random value taken from
stack)


[PATCH v3 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. 

If we detect a problematic load M(K), we replace it by a load of
immediate value 0

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Dan Rosenberg <drosenberg@vsecurity.com>
Cc: Changli Gao <xiaosuo@gmail.com>
---
v3: replace problematic loads M(K) by load of immediate 0 value,
    dont report an error to user.
v2: set memvalid to ~0 on JMP instructions

 net/core/filter.c |   78 ++++++++++++++++++++++++++++++++++++++------
 1 file changed, 69 insertions(+), 9 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index a44d27f..4456a6c 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 +413,72 @@ 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.
+ * If such malicious (or buggy) read is detected, its replaced by a
+ * load of immediate zero value.
+ */
+static int check_load_and_stores(struct sock_filter *filter, int flen)
+{
+	u16 *masks, memvalid = 0; /* one bit per cell, 16 cells */
+	int pc;
+
+	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:
+			if (!(memvalid & (1 << filter[pc].k))) {
+				filter[pc].code = BPF_S_LD_IMM;
+				filter[pc].k = 0;
+			}
+			break;
+		case BPF_S_LDX_MEM:
+			if (!(memvalid & (1 << filter[pc].k))) {
+				filter[pc].code = BPF_S_LDX_IMM;
+				filter[pc].k = 0;
+			}
+			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;
+		}
+	}
+	kfree(masks);
+	return 0;
+}
+
 /**
  *	sk_chk_filter - verify socket filter code
  *	@filter: filter to verify
@@ -547,7 +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;
 }