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[46.193.1.135]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4872712b3ebsm20378445e9.19.2026.03.27.13.41.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 27 Mar 2026 13:41:58 -0700 (PDT) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Fri, 27 Mar 2026 21:41:56 +0100 To: Leon Hwang Cc: bpf@vger.kernel.org, Alexei Starovoitov , Daniel Borkmann , John Fastabend , Andrii Nakryiko , Martin KaFai Lau , Eduard Zingerman , Song Liu , Yonghong Song , KP Singh , Stanislav Fomichev , Hao Luo , Shuah Khan , "David S . Miller" , Jakub Kicinski , Jesper Dangaard Brouer , Toke Hoiland-Jorgensen , Lorenzo Bianconi , linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, netdev@vger.kernel.org, kernel-patches-bot@fb.com Subject: Re: [PATCH bpf-next 2/3] bpf: Disallow freplace on kprobe with mismatched kprobe_write_ctx values Message-ID: References: <20260324150444.68166-1-leon.hwang@linux.dev> <20260324150444.68166-3-leon.hwang@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260324150444.68166-3-leon.hwang@linux.dev> 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 > --- > 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