[PATCH bpf v1 5/8] bpf: Mark NULL kptr stores precise

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

check_map_kptr_access() permits a scalar store into an untrusted kptr
field only when the register is known to contain zero. Unlike other
verifier checks whose outcome depends on a scalar value, it does not mark
that register precise.

A state checkpoint reached with an imprecise zero can therefore prune a
second path that reaches the store with an arbitrary nonzero scalar. The
program can write attacker-controlled bits into the kptr field and load
them back as a PTR_TO_BTF_ID.

Call mark_chain_precision() before accepting a known-zero register. This
forces state equivalence to compare its scalar range and makes the verifier
visit and reject a path carrying a nonzero value.

Fixes: 61df10c7799e ("bpf: Allow storing unreferenced kptr in map")
Reported-by: Nicholas Carlini <npc@anthropic.com>
Suggested-by: Nicholas Carlini <npc@anthropic.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/verifier.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index d7dd0befbd10..ba9753bf529a 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4694,8 +4694,15 @@ static int check_map_kptr_access(struct bpf_verifier_env *env,
 			return ret;
 	} else if (class == BPF_STX) {
 		val_reg = reg_state(env, value_regno);
-		if (!bpf_register_is_null(val_reg) &&
-		    map_kptr_match_type(env, kptr_field, val_reg, value_regno))
+		if (bpf_register_is_null(val_reg)) {
+			/*
+			 * This store is valid only because the scalar is known to be
+			 * zero. Mark it precise so another scalar cannot be pruned
+			 * against this state.
+			 */
+			return mark_chain_precision(env, value_regno);
+		}
+		if (map_kptr_match_type(env, kptr_field, val_reg, value_regno))
 			return -EACCES;
 	} else if (class == BPF_ST) {
 		if (insn->imm) {
-- 
2.53.0