[PATCH bpf-next v1 3/6] bpf: Disallow negative offset in check_ptr_off_reg

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

check_ptr_off_reg only allows fixed offset to be set for PTR_TO_BTF_ID,
where reg->off < 0 doesn't make sense. This would shift the pointer
backwards, and fails later in btf_struct_ids_match or btf_struct_walk
due to out of bounds access (since offset is interpreted as unsigned).

Improve the verifier by rejecting this case by using a better error
message for BPF helpers and kfunc, by putting a check inside the
check_func_arg_reg_off function.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/verifier.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index a641e61767b4..9f12a343bb6e 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3984,6 +3984,12 @@ static int __check_ptr_off_reg(struct bpf_verifier_env *env,
 	 * is only allowed in its original, unmodified form.
 	 */
 
+	if (reg->off < 0) {
+		verbose(env, "negative offset %s ptr R%d off=%d disallowed\n",
+			reg_type_str(env, reg->type), regno, reg->off);
+		return -EACCES;
+	}
+
 	if (!fixed_off_ok && reg->off) {
 		verbose(env, "dereference of modified %s ptr R%d off=%d disallowed\n",
 			reg_type_str(env, reg->type), regno, reg->off);
-- 
2.35.1