Re: [PATCH bpf 1/2] bpf: Reject >8 byte return values on return-reading trampoline paths

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] The x86 BPF trampoline generation under-allocates stack space for 16-byte scalar arguments (e.g., __int128), leading to an out-of-bounds stack write that overwrites the saved RBP and return address.
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--- Patch [1]: [PATCH bpf 1/2] bpf: Reject >8 byte return values on return-reading trampoline paths ---
commit 8947b8106804bc7e7328c045366441fc0d165459
Author: Yonghong Song <yonghong.song@linux.dev>

bpf: Reject >8 byte return values on return-reading trampoline paths

This patch rejects BPF trampoline attachments for types like fexit, fmod_ret,
and struct_ops if the target function has a return value larger than 8 bytes.
Previously, the trampoline accepted 16-byte return values (e.g., __int128) but
only saved and restored 8 bytes, which corrupted the return value seen by the
caller.

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 6515d4d3c0032..26d281b77a6e7 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c

[ ... ]

> @@ -19137,6 +19151,14 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
>  		if (ret < 0)
>  			return ret;
>  
> +		if (tgt_info->fmodel.ret_size > 8 &&
> +		    attach_uses_trampoline_retval(prog->expected_attach_type)) {
[Severity: High]
This is a pre-existing issue, but while this patch addresses >8 byte return
values, does the BPF trampoline also have a similar problem with >8 byte
arguments?

In kernel/bpf/btf.c:btf_distill_func_proto(), 16-byte arguments like __int128
are accepted, but __get_type_fmodel_flags() does not set BTF_FMODEL_STRUCT_ARG
for them because they are BTF_KIND_INT, not structs.

In arch/x86/net/bpf_jit_comp.c:__arch_prepare_bpf_trampoline(), the stack
size calculation relies on this flag:

		if (m->arg_flags[i] & BTF_FMODEL_STRUCT_ARG)
			nr_regs += (m->arg_size[i] + 7) / 8 - 1;

Because the flag isn't set for __int128, it is incorrectly counted as
1 register, under-allocating the stack space by 8 bytes per 16-byte
scalar argument.

Later during code generation, save_args() computes
arg_regs = (m->arg_size[i] + 7) / 8, which evaluates to 2. It performs
two 8-byte stack stores, which will underflow the allocated boundary and
overwrite the saved RBP and potentially the return address.

Could this lead to out-of-bounds stack writes for fentry/fexit programs
attached to functions with 16-byte scalar arguments?

> +			bpf_log(log,
> +				"Attach to function %s with a >8 byte return value is not supported for this attach type\n",
> +				tname);
> +			return -EOPNOTSUPP;
> +		}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260710144404.2579671-1-yonghong.song@linux.dev?part=1