filter: make sure filters dont read uninitialized memory
David S. Miller [Wed, 10 Nov 2010 18:38:24 +0000 (10:38 -0800)]
There is a possibility malicious users can get limited information about
uninitialized stack mem array. Even if sk_run_filter() result is bound
to packet length (0 .. 65535), we could imagine this can be used by
hostile user.

Initializing mem[] array, like Dan Rosenberg suggested in his patch is
expensive since most filters dont even use this array.

Its hard to make the filter validation in sk_chk_filter(), because of
the jumps. This might be done later.

In this patch, I use a bitmap (a single long var) so that only filters
using mem[] loads/stores pay the price of added security checks.

For other filters, additional cost is a single instruction.

[ Since we access fentry->k a lot now, cache it in a local variable
  and mark filter entry pointer as const. -DaveM ]

Reported-by: Dan Rosenberg <drosenberg@vsecurity.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/filter.c

index 7beaec3..23e9b2a 100644 (file)
@@ -112,39 +112,41 @@ EXPORT_SYMBOL(sk_filter);
  */
 unsigned int sk_run_filter(struct sk_buff *skb, struct sock_filter *filter, int flen)
 {
-       struct sock_filter *fentry;     /* We walk down these */
        void *ptr;
        u32 A = 0;                      /* Accumulator */
        u32 X = 0;                      /* Index Register */
        u32 mem[BPF_MEMWORDS];          /* Scratch Memory Store */
+       unsigned long memvalid = 0;
        u32 tmp;
        int k;
        int pc;
 
+       BUILD_BUG_ON(BPF_MEMWORDS > BITS_PER_LONG);
        /*
         * Process array of filter instructions.
         */
        for (pc = 0; pc < flen; pc++) {
-               fentry = &filter[pc];
+               const struct sock_filter *fentry = &filter[pc];
+               u32 f_k = fentry->k;
 
                switch (fentry->code) {
                case BPF_S_ALU_ADD_X:
                        A += X;
                        continue;
                case BPF_S_ALU_ADD_K:
-                       A += fentry->k;
+                       A += f_k;
                        continue;
                case BPF_S_ALU_SUB_X:
                        A -= X;
                        continue;
                case BPF_S_ALU_SUB_K:
-                       A -= fentry->k;
+                       A -= f_k;
                        continue;
                case BPF_S_ALU_MUL_X:
                        A *= X;
                        continue;
                case BPF_S_ALU_MUL_K:
-                       A *= fentry->k;
+                       A *= f_k;
                        continue;
                case BPF_S_ALU_DIV_X:
                        if (X == 0)
@@ -152,49 +154,49 @@ unsigned int sk_run_filter(struct sk_buff *skb, struct sock_filter *filter, int
                        A /= X;
                        continue;
                case BPF_S_ALU_DIV_K:
-                       A /= fentry->k;
+                       A /= f_k;
                        continue;
                case BPF_S_ALU_AND_X:
                        A &= X;
                        continue;
                case BPF_S_ALU_AND_K:
-                       A &= fentry->k;
+                       A &= f_k;
                        continue;
                case BPF_S_ALU_OR_X:
                        A |= X;
                        continue;
                case BPF_S_ALU_OR_K:
-                       A |= fentry->k;
+                       A |= f_k;
                        continue;
                case BPF_S_ALU_LSH_X:
                        A <<= X;
                        continue;
                case BPF_S_ALU_LSH_K:
-                       A <<= fentry->k;
+                       A <<= f_k;
                        continue;
                case BPF_S_ALU_RSH_X:
                        A >>= X;
                        continue;
                case BPF_S_ALU_RSH_K:
-                       A >>= fentry->k;
+                       A >>= f_k;
                        continue;
                case BPF_S_ALU_NEG:
                        A = -A;
                        continue;
                case BPF_S_JMP_JA:
-                       pc += fentry->k;
+                       pc += f_k;
                        continue;
                case BPF_S_JMP_JGT_K:
-                       pc += (A > fentry->k) ? fentry->jt : fentry->jf;
+                       pc += (A > f_k) ? fentry->jt : fentry->jf;
                        continue;
                case BPF_S_JMP_JGE_K:
-                       pc += (A >= fentry->k) ? fentry->jt : fentry->jf;
+                       pc += (A >= f_k) ? fentry->jt : fentry->jf;
                        continue;
                case BPF_S_JMP_JEQ_K:
-                       pc += (A == fentry->k) ? fentry->jt : fentry->jf;
+                       pc += (A == f_k) ? fentry->jt : fentry->jf;
                        continue;
                case BPF_S_JMP_JSET_K:
-                       pc += (A & fentry->k) ? fentry->jt : fentry->jf;
+                       pc += (A & f_k) ? fentry->jt : fentry->jf;
                        continue;
                case BPF_S_JMP_JGT_X:
                        pc += (A > X) ? fentry->jt : fentry->jf;
@@ -209,7 +211,7 @@ unsigned int sk_run_filter(struct sk_buff *skb, struct sock_filter *filter, int
                        pc += (A & X) ? fentry->jt : fentry->jf;
                        continue;
                case BPF_S_LD_W_ABS:
-                       k = fentry->k;
+                       k = f_k;
 load_w:
                        ptr = load_pointer(skb, k, 4, &tmp);
                        if (ptr != NULL) {
@@ -218,7 +220,7 @@ load_w:
                        }
                        break;
                case BPF_S_LD_H_ABS:
-                       k = fentry->k;
+                       k = f_k;
 load_h:
                        ptr = load_pointer(skb, k, 2, &tmp);
                        if (ptr != NULL) {
@@ -227,7 +229,7 @@ load_h:
                        }
                        break;
                case BPF_S_LD_B_ABS:
-                       k = fentry->k;
+                       k = f_k;
 load_b:
                        ptr = load_pointer(skb, k, 1, &tmp);
                        if (ptr != NULL) {
@@ -242,32 +244,34 @@ load_b:
                        X = skb->len;
                        continue;
                case BPF_S_LD_W_IND:
-                       k = X + fentry->k;
+                       k = X + f_k;
                        goto load_w;
                case BPF_S_LD_H_IND:
-                       k = X + fentry->k;
+                       k = X + f_k;
                        goto load_h;
                case BPF_S_LD_B_IND:
-                       k = X + fentry->k;
+                       k = X + f_k;
                        goto load_b;
                case BPF_S_LDX_B_MSH:
-                       ptr = load_pointer(skb, fentry->k, 1, &tmp);
+                       ptr = load_pointer(skb, f_k, 1, &tmp);
                        if (ptr != NULL) {
                                X = (*(u8 *)ptr & 0xf) << 2;
                                continue;
                        }
                        return 0;
                case BPF_S_LD_IMM:
-                       A = fentry->k;
+                       A = f_k;
                        continue;
                case BPF_S_LDX_IMM:
-                       X = fentry->k;
+                       X = f_k;
                        continue;
                case BPF_S_LD_MEM:
-                       A = mem[fentry->k];
+                       A = (memvalid & (1UL << f_k)) ?
+                               mem[f_k] : 0;
                        continue;
                case BPF_S_LDX_MEM:
-                       X = mem[fentry->k];
+                       X = (memvalid & (1UL << f_k)) ?
+                               mem[f_k] : 0;
                        continue;
                case BPF_S_MISC_TAX:
                        X = A;
@@ -276,14 +280,16 @@ load_b:
                        A = X;
                        continue;
                case BPF_S_RET_K:
-                       return fentry->k;
+                       return f_k;
                case BPF_S_RET_A:
                        return A;
                case BPF_S_ST:
-                       mem[fentry->k] = A;
+                       memvalid |= 1UL << f_k;
+                       mem[f_k] = A;
                        continue;
                case BPF_S_STX:
-                       mem[fentry->k] = X;
+                       memvalid |= 1UL << f_k;
+                       mem[f_k] = X;
                        continue;
                default:
                        WARN_ON(1);