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>

net/core/filter.c

@@ -295,42 +295,42 @@ unsigned int __sk_run_filter(void *ctx, const struct sock_filter_int *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);
 		CONT;
 	BPF_ALU_BPF_MOD_BPF_X:
+		if (unlikely(X == 0))
+			return 0;
 		tmp = (u32) A;
-		if (X)
 		A = do_div(tmp, (u32) X);
 		CONT;
 	BPF_ALU64_BPF_MOD_BPF_K:
 		tmp = A;
-		if (K)
 		A = do_div(tmp, K);
 		CONT;
 	BPF_ALU_BPF_MOD_BPF_K:
 		tmp = (u32) A;
-		if (K)
 		A = do_div(tmp, (u32) K);
 		CONT;
 	BPF_ALU64_BPF_DIV_BPF_X:
-		if (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);
 		A = (u32) tmp;
 		CONT;
 	BPF_ALU64_BPF_DIV_BPF_K:
-		if (K)
 		do_div(A, K);
 		CONT;
 	BPF_ALU_BPF_DIV_BPF_K:
 		tmp = (u32) A;
-		if (K)
 		do_div(tmp, (u32) K);
 		A = (u32) tmp;
 		CONT;