[PATCH] bpf: reject stack-argument callback subprograms

[PATCH] bpf: reject stack-argument callback subprograms

[Jérémy Jean]

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 <Jeremy.Jean@oss.cyber.gouv.fr>
---
 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

Re: [PATCH] bpf: reject stack-argument callback subprograms

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 2429 bytes --]

> 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.

This isn't a bug, but would it be worth extending this paragraph to cover
argument six as well?  The new check rejects everything above five args,
and arch/x86/net/bpf_jit_comp.c documents that incoming arg 6 is read from
R9, which similarly doesn't match the value the verifier believes the
caller stored at r11-8.

[ ... ]

> 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
> @@ -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
> +	);
> +}

This isn't a bug, but could the unused 'r6 = 0' be dropped?  It's written
but never read, and the asm clobber list is __clobber_all.

Also, would a one-line comment on the four r11 stores help the next
reader?  They're what makes the outgoing stack-arg slots initialized so
the new check is the one that fires.


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/31972309086

Re: [PATCH] bpf: reject stack-argument callback subprograms

[Pu Lehui]

+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 <Jeremy.Jean@oss.cyber.gouv.fr>
> ---
>   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

Re: [PATCH] bpf: reject stack-argument callback subprograms

[Jérémy Jean]

On 2026-08-17 04:16, Pu Lehui wrote:
> 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.

Thanks for the feedback.
Do you want me to do the split, or will you do it?

Jérémy

Re: [PATCH] bpf: reject stack-argument callback subprograms

[Daniel Borkmann]

Hi Jérémy,

On 8/17/26 12:04 PM, Jérémy Jean wrote:
> On 2026-08-17 04:16, Pu Lehui wrote:
>> 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.
> 
> Thanks for the feedback.
> Do you want me to do the split, or will you do it?
Please send a v2 with the split, so it can go through BPF CI again.

Thanks,
Daniel

Re: [PATCH] bpf: reject stack-argument callback subprograms

[Yonghong Song]

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 <Jeremy.Jean@oss.cyber.gouv.fr>
> ---
>   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