[PATCH bpf-next 4/7] bpf: fix check for attempt to corrupt spilled pointer

Andrii Nakryiko
2023-10-31T05:03:45Z

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 098ba0e1a6ff..82992c32c1bd 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4622,7 +4622,7 @@ 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