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>

1 files changed, 63 insertions, 9 deletions

diff --git a/net/core/filter.c b/net/core/filter.c
index 054e286861d2..ac4920a87be5 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:
@@ -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;
 }