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Date:   Sat, 19 Sep 2020 13:49:44 +0200
From:   Toke Høiland-Jørgensen <>
To:     Alexei Starovoitov <>
Cc:     Daniel Borkmann <>,
        Martin KaFai Lau <>,
        Song Liu <>, Yonghong Song <>,
        Andrii Nakryiko <>,
        John Fastabend <>,
        Jiri Olsa <>,
        Eelco Chaudron <>,
        KP Singh <>,,
Subject: [PATCH bpf-next v7 01/10] bpf: disallow attaching modify_return
 tracing functions to other BPF programs

From: Toke Høiland-Jørgensen <>

>From the checks and commit messages for modify_return, it seems it was
never the intention that it should be possible to attach a tracing program
with expected_attach_type == BPF_MODIFY_RETURN to another BPF program.
However, check_attach_modify_return() will only look at the function name,
so if the target function starts with "security_", the attach will be
allowed even for bpf2bpf attachment.

Fix this oversight by also blocking the modification if a target program is

Fixes: 18644cec714a ("bpf: Fix use-after-free in fmod_ret check")
Fixes: 6ba43b761c41 ("bpf: Attachment verification for BPF_MODIFY_RETURN")
Signed-off-by: Toke Høiland-Jørgensen <>
 kernel/bpf/verifier.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 4161b6c406bc..cb1b0f9fd770 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11442,7 +11442,7 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
 		} else if (prog->expected_attach_type == BPF_MODIFY_RETURN) {
 			ret = check_attach_modify_return(prog, addr);
-			if (ret)
+			if (ret || tgt_prog)
 				verbose(env, "%s() is not modifiable\n",

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