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bpf: Mark NULL kptr stores precise

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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: 61df10c779 ("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>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260904104203.345917-6-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Kumar Kartikeya Dwivedi 2026-09-04 12:41:56 +02:00 committed by Alexei Starovoitov
parent 2b97956af6
commit ecdc504379
1 changed files with 9 additions and 2 deletions
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11
kernel/bpf/verifier.c
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@ -4708,8 +4708,15 @@ 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) {