[PATCH bpf-next v1 04/25] bpf: Fix slot type check in check_stack_write_var_off

Kumar Kartikeya Dwivedi
2022-10-11T01:23:45Z

For the case where allow_ptr_leaks is false, code is checking whether
slot type is STACK_INVALID and STACK_SPILL and rejecting other cases.
This is a consequence of incorrectly checking for register type instead
of the slot type (NOT_INIT and SCALAR_VALUE respectively). Fix the
check.

Fixes: 01f810ace9ed ("bpf: Allow variable-offset stack access")
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
kernel/bpf/verifier.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 48a10d79f1bf..bbbd44b0fd6f 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3181,14 +3181,17 @@ static int check_stack_write_var_off(struct bpf_verifier_env *env,
stype = &state->stack[spi].slot_type[slot % BPF_REG_SIZE];
mark_stack_slot_scratched(env, spi);

- if (!env->allow_ptr_leaks
- && *stype != NOT_INIT
- && *stype != SCALAR_VALUE) {
- /* Reject the write if there's are spilled pointers in
- * range. If we didn't reject here, the ptr status
- * would be erased below (even though not all slots are
- * actually overwritten), possibly opening the door to
- * leaks.
+ if (!env->allow_ptr_leaks && *stype != STACK_MISC && *stype != STACK_ZERO) {
+ /* Reject the write if range we may write to has not
+ * been initialized beforehand. If we didn't reject
+ * here, the ptr status would be erased below (even
+ * though not all slots are actually overwritten),
+ * possibly opening the door to leaks.
+ *
+ * We do however catch STACK_INVALID case below, and
+ * only allow reading possibly uninitialized memory
+ * later for CAP_PERFMON, as the write may not happen to
+ * that slot.
*/
verbose(env, "spilled ptr in range of var-offset stack write; insn %d, ptr off: %d",
insn_idx, i);
--
2.34.1