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| author | Kumar Kartikeya Dwivedi <memxor@gmail.com> | 2026-09-04 12:41:56 +0200 |
|---|---|---|
| committer | Alexei Starovoitov <ast@kernel.org> | 2026-09-04 12:24:25 -0700 |
| commit | ecdc5043794c9184aa8e6c814603899479c46b35 (patch) | |
| tree | da2d125a692419990c1e0564e0f97f8a8c265c3b | |
| parent | 2b97956af60810cd382b86b9ce9aea421b889861 (diff) | |
| download | linux-ecdc5043794c9184aa8e6c814603899479c46b35.tar.gz linux-ecdc5043794c9184aa8e6c814603899479c46b35.zip | |
bpf: Mark NULL kptr stores precise
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>
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>
| -rw-r--r-- | kernel/bpf/verifier.c | 11 |
1 files changed, 9 insertions, 2 deletions
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 9c6ad157a61e..b71c5274b3dc 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -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) { |