[PATCH] bpf: reject stack-argument callback subprograms

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;
+	}
 
 	/* 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
-- 
2.47.3