net: filter: be more defensive on div/mod by X==0

The old interpreter behaviour was that we returned with 0
whenever we found a division by 0 would take place. In the new
interpreter we would currently just skip that instead and
continue execution.

It's true that a value of 0 as return might not be appropriate
in all cases, but current users (socket filters -> drop
packet, seccomp -> SECCOMP_RET_KILL, cls_bpf -> unclassified,
etc) seem fine with that behaviour. Better this than undefined
BPF program behaviour as it's expected that A contains the
result of the division. In future, as more use cases open up,
we could further adapt this return value to our needs, if
necessary.

So reintroduce return of 0 for division by 0 as in the old
interpreter. Also in case of K which is guaranteed to be 32bit
wide, sk_chk_filter() already takes care of preventing division
by 0 invoked through K, so we can generally spare us these tests.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Reviewed-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 16 insertions, 16 deletions

diff --git a/net/core/filter.c b/net/core/filter.c
index 765556ba32ef..e08b3822c72a 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -295,43 +295,43 @@ select_insn:
                 (*(s64 *) &A) >>= K;
                 CONT;
         BPF_ALU64_BPF_MOD_BPF_X:
+                if (unlikely(X == 0))
+                        return 0;
                 tmp = A;
-                if (X)
-                        A = do_div(tmp, X);
+                A = do_div(tmp, X);
                 CONT;
         BPF_ALU_BPF_MOD_BPF_X:
+                if (unlikely(X == 0))
+                        return 0;
                 tmp = (u32) A;
-                if (X)
-                        A = do_div(tmp, (u32) X);
+                A = do_div(tmp, (u32) X);
                 CONT;
         BPF_ALU64_BPF_MOD_BPF_K:
                 tmp = A;
-                if (K)
-                        A = do_div(tmp, K);
+                A = do_div(tmp, K);
                 CONT;
         BPF_ALU_BPF_MOD_BPF_K:
                 tmp = (u32) A;
-                if (K)
-                        A = do_div(tmp, (u32) K);
+                A = do_div(tmp, (u32) K);
                 CONT;
         BPF_ALU64_BPF_DIV_BPF_X:
-                if (X)
-                        do_div(A, X);
+                if (unlikely(X == 0))
+                        return 0;
+                do_div(A, X);
                 CONT;
         BPF_ALU_BPF_DIV_BPF_X:
+                if (unlikely(X == 0))
+                        return 0;
                 tmp = (u32) A;
-                if (X)
-                        do_div(tmp, (u32) X);
+                do_div(tmp, (u32) X);
                 A = (u32) tmp;
                 CONT;
         BPF_ALU64_BPF_DIV_BPF_K:
-                if (K)
-                        do_div(A, K);
+                do_div(A, K);
                 CONT;
         BPF_ALU_BPF_DIV_BPF_K:
                 tmp = (u32) A;
-                if (K)
-                        do_div(tmp, (u32) K);
+                do_div(tmp, (u32) K);
                 A = (u32) tmp;
                 CONT;
         BPF_ALU_BPF_END_BPF_TO_BE: