[PATCH bpf v2 7/9] bpf: fix check_map_access smin_value test when pointer contains offset

[Daniel Borkmann <daniel@iogearbox.net>]

In check_map_access() we probe actual bounds through __check_map_access()
with offset of reg->smin_value + off for lower bound and offset of
reg->umax_value + off for the upper bound. However, even though the
reg->smin_value could have a negative value, the final result of the
sum with off could be positive when pointer arithmetic with known and
unknown scalars is combined. In this case we reject the program with
an error such as "R<x> min value is negative, either use unsigned index
or do a if (index >=0) check." even though the access itself would be
fine. Therefore extend the check to probe whether the actual resulting
reg->smin_value + off is less than zero.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
---
 kernel/bpf/verifier.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index eebbc03..8e5da1c 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1443,13 +1443,17 @@ static int check_map_access(struct bpf_verifier_env *env, u32 regno,
 	 */
 	if (env->log.level)
 		print_verifier_state(env, state);
+
 	/* The minimum value is only important with signed
 	 * comparisons where we can't assume the floor of a
 	 * value is 0.  If we are using signed variables for our
 	 * index'es we need to make sure that whatever we use
 	 * will have a set floor within our range.
 	 */
-	if (reg->smin_value < 0) {
+	if (reg->smin_value < 0 &&
+	    (reg->smin_value == S64_MIN ||
+	     (off + reg->smin_value != (s64)(s32)(off + reg->smin_value)) ||
+	      reg->smin_value + off < 0)) {
 		verbose(env, "R%d min value is negative, either use unsigned index or do a if (index >=0) check.\n",
 			regno);
 		return -EACCES;
-- 
2.9.5