+cc Yonghong On 2026/8/17 4:45, Jérémy Jean wrote:
Helper callbacks enter BPF subprograms through bpf_callback_t, whose runtime ABI supplies five arguments. BTF validation nevertheless permits static callback subprograms to declare more than five arguments when JIT stack arguments are supported.This lets verifier state for a callback use outgoing stack argument slots prepared at the helper call site. The helper does not pass those slots. On x86-64, callback loads of arguments seven and later therefore read the helper native frame instead of the synthetic values checked by the verifier. KASAN reports a slab OOB write. Reject callback subprograms with incoming stack arguments when processing callback calls. Add a verifier regression test using bpf_loop() and a nine-argument callback. Fixes: 0f6bd5e7a804 ("bpf: Support stack arguments for bpf functions") Assisted-by: Codex:gpt-5 Signed-off-by: Jérémy Jean <[email protected]> --- kernel/bpf/verifier.c | 4 +++ .../selftests/bpf/progs/verifier_stack_arg.c | 33 +++++++++++++++++++ 2 files changed, 37 insertions(+) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index fdc5fbb1f78c..29aa4911c7f7 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -9285,6 +9285,10 @@ static int push_callback_call(struct bpf_verifier_env *env, struct bpf_insn *ins err = btf_check_subprog_call(env, subprog, caller->regs); if (err == -EFAULT) return err; + if (bpf_in_stack_arg_cnt(&env->subprog_info[subprog])) { + verbose(env, "callback subprog cannot have stack args\n"); + return -EINVAL; + }
Just took a look at stack args feature. The limit looks good to me, but it would be better to split the fix and selftest into two separate patches.
/* set_callee_state is used for direct subprog calls, but we are* interested in validating only BPF helpers that can call subprogs as diff --git a/tools/testing/selftests/bpf/progs/verifier_stack_arg.c b/tools/testing/selftests/bpf/progs/verifier_stack_arg.c index 7e0ce5db28a0..5acc3b63ca84 100644 --- a/tools/testing/selftests/bpf/progs/verifier_stack_arg.c +++ b/tools/testing/selftests/bpf/progs/verifier_stack_arg.c @@ -27,6 +27,13 @@ static int subprog_7args(int a, int b, int c, int d, int e, int f, int g) return a + b + c + d + e + f + g; }+__noinline __used+static int callback_9args(__u32 index, void *ctx, long a3, long a4, + long a5, long a6, long a7, long a8, long a9) +{ + return a9; +} + __noinline __used static long subprog_deref_arg6(long a, long b, long c, long d, long e, long *f) { @@ -79,6 +86,32 @@ __naked void stack_arg_two_subprogs(void) ); }+SEC("tc")+__description("stack_arg: callback with incoming stack args") +__failure +__msg("callback subprog cannot have stack args") +__naked void stack_arg_callback_many_args(void) +{ + asm volatile ( + "r6 = 0;" + "*(u64 *)(r11 - 32) = 0;" + "*(u64 *)(r11 - 24) = 0;" + "*(u64 *)(r11 - 16) = 0;" + "*(u64 *)(r11 - 8) = 0;" + "r1 = 1;" + "r2 = %[callback_9args];" + "r3 = 0;" + "r4 = 0;" + "call %[bpf_loop];" + "r0 = 0;" + "exit;" + : + : __imm_ptr(callback_9args), + __imm(bpf_loop) + : __clobber_all + ); +} + SEC("tc") __description("stack_arg: read from uninitialized stack arg slot") __failure
