[PATCH v3 bpf-next 03/10] bpf: fix check for attempt to corrupt spilled pointer

[Andrii Nakryiko <andrii@kernel.org>]

When register is spilled onto a stack as a 1/2/4-byte register, we set
slot_type[BPF_REG_SIZE - 1] (plus potentially few more below it,
depending on actual spill size). So to check if some stack slot has
spilled register we need to consult slot_type[7], not slot_type[0].

To avoid the need to remember and double-check this in the future, just
use is_spilled_reg() helper.

Fixes: 638f5b90d460 ("bpf: reduce verifier memory consumption")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 kernel/bpf/verifier.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 4f8a3c77eb80..73315e2f20d9 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4431,7 +4431,7 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
 	 * so it's aligned access and [off, off + size) are within stack limits
 	 */
 	if (!env->allow_ptr_leaks &&
-	    state->stack[spi].slot_type[0] == STACK_SPILL &&
+	    is_spilled_reg(&state->stack[spi]) &&
 	    size != BPF_REG_SIZE) {
 		verbose(env, "attempt to corrupt spilled pointer on stack\n");
 		return -EACCES;
-- 
2.34.1