Subject: Re: [PATCH bpf-next 2/3] bpf: Disallow freplace on kprobe with mismatched kprobe_write_ctx values

From: Jiri Olsa <[email-masked]>
Date: Fri, 27 Mar 2026 21:41:56 +0100

On Tue, Mar 24, 2026 at 11:04:43PM +0800, Leon Hwang wrote:
> uprobe programs are allowed to modify struct pt_regs.
> 
> Since the actual program type of uprobe is KPROBE, it can be abused to
> modify struct pt_regs via kprobe+freplace when the kprobe attaches to
> kernel functions.
> 
> For example,
> 
> SEC("?kprobe")
> int kprobe(struct pt_regs *regs)
> {
> 	return 0;
> }
> 
> SEC("?freplace")
> int freplace_kprobe(struct pt_regs *regs)
> {
> 	regs->di = 0;
> 	return 0;
> }
> 
> freplace_kprobe prog will attach to kprobe prog.
> kprobe prog will attach to a kernel function.
> 
> Without this patch, when the kernel function runs, its first arg will
> always be set as 0 via the freplace_kprobe prog.
> 
> To avoid the abuse of kprobe_write_ctx=true via kprobe+freplace, disallow
> freplace on kprobe programs with mismatched kprobe_write_ctx values.
> 
> Fixes: 7384893d970e ("bpf: Allow uprobe program to change context registers")
> Signed-off-by: Leon Hwang <[email-masked]>
> ---
>  kernel/bpf/verifier.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 12330466d58b..f8257bae6081 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -6404,6 +6404,14 @@ static int check_ctx_access(struct bpf_verifier_env *env, int insn_idx, int off,
>  		/* remember the offset of last byte accessed in ctx */
>  		if (env->prog->aux->max_ctx_offset < off + size)
>  			env->prog->aux->max_ctx_offset = off + size;
> +		if (env->prog->type == BPF_PROG_TYPE_EXT) {
> +			struct bpf_prog *dst_prog = env->prog->aux->dst_prog;
> +
> +			if (env->prog->aux->kprobe_write_ctx != dst_prog->aux->kprobe_write_ctx) {
> +				verbose(env, "Extension program cannot have different kprobe_write_ctx value with target prog\n");
> +				return -EINVAL;
> +			}
> +		}

would it be more clear to check this in bpf_check_attach_target,
instead of depending on actual write to context

jirka