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filter: make sure filters dont read uninitialized memory
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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>
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David S. Miller committed Nov 10, 2010
1 parent fe10ae5 commit 57fe93b
Showing 1 changed file with 35 additions and 29 deletions.
64 changes: 35 additions & 29 deletions net/core/filter.c
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Original file line number Diff line number Diff line change
Expand Up @@ -112,89 +112,91 @@ 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)
return 0;
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;
Expand All @@ -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) {
Expand All @@ -218,7 +220,7 @@ unsigned int sk_run_filter(struct sk_buff *skb, struct sock_filter *filter, int
}
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) {
Expand All @@ -227,7 +229,7 @@ unsigned int sk_run_filter(struct sk_buff *skb, struct sock_filter *filter, int
}
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) {
Expand All @@ -242,32 +244,34 @@ unsigned int sk_run_filter(struct sk_buff *skb, struct sock_filter *filter, int
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;
Expand All @@ -276,14 +280,16 @@ unsigned int sk_run_filter(struct sk_buff *skb, struct sock_filter *filter, int
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);
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