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. (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>

diff --git a/net/core/filter.c b/net/core/filter.c
index 054e286..ac4920a 100644 (file)
--- 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:
@@ -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;
 }