On 8/16/26 1:45 PM, 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;
+ }
/* 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
+ );
+}
I build with latest bpf-next and selftest with llvm23 and I got a selftest build
failure:
progs/verifier_stack_arg.c:96:3: error: inline assembly requires more registers
than available
96 | "r6 = 0;"
| ^
CLNG-BPF [test_progs] verifier_value_or_null.bpf.o
1 error generated.
The reason probably due to '__imm_ptr(callback_9args)'. The following change
can fix the build:
index 5acc3b63ca84..f55a433b9202 100644
--- a/tools/testing/selftests/bpf/progs/verifier_stack_arg.c
+++ b/tools/testing/selftests/bpf/progs/verifier_stack_arg.c
@@ -108,7 +108,7 @@ __naked void stack_arg_callback_many_args(void)
:
: __imm_ptr(callback_9args),
__imm(bpf_loop)
- : __clobber_all
+ : __clobber_common, "r6"
);
}
I then tested without your verifier.c change, I get the following test failure:
All error logs:
tester_init:PASS:tester_log_buf 0 nsec
process_subtest:PASS:obj_open_mem 0 nsec
process_subtest:PASS:specs_alloc 0 nsec
run_subtest:PASS:obj_open_mem 0 nsec
libbpf: prog 'stack_arg_callback_many_args': BPF program load failed:
-EINVAL
libbpf: prog 'stack_arg_callback_many_args': failed to load: -EINVAL
libbpf: failed to load object 'verifier_stack_arg'
run_subtest:PASS:unexpected_load_success 0 nsec
validate_msgs:FAIL:979 expect_msg
VERIFIER LOG:
=============
func#0 writes 4 stack arg slots, but calls only require 0
processed 21 insns (limit 1000000) max_states_per_insn 1 total_states 2
peak_states 2 mark_read 0
=============
EXPECTED SUBSTR: 'callback subprog cannot have stack args'
Summary: 1/18 PASSED, 0 SKIPPED, 1/1 FAILED
I think the error message "func#0 writes 4 stack arg slots, but calls only require
0"
is good enough. So the extra error message "callback subprog cannot have stack
args"
is not necessary in my opinion.
+
SEC("tc")
__description("stack_arg: read from uninitialized stack arg slot")
__failure
