From: =?UTF-8?q?Toke=20H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
Date: Fri, 25 Sep 2020 23:25:00 +0200
Subject: bpf: disallow attaching modify_return tracing functions to other BPF
 programs
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Patch-mainline: Queued in subsystem maintainer repository
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
Git-commit: 1af9270e908cd50a4f5d815c9b6f794c7d57ed07
References: bsc#1177028

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
supplied.

Fixes: 18644cec714a ("bpf: Fix use-after-free in fmod_ret check")
Fixes: 6ba43b761c41 ("bpf: Attachment verification for BPF_MODIFY_RETURN")
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Gary Lin <glin@suse.com>
---
 kernel/bpf/verifier.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10642,6 +10642,11 @@ static int check_attach_btf_id(struct bp
 		}
 
 		if (prog->expected_attach_type == BPF_MODIFY_RETURN) {
+			if (tgt_prog) {
+				verbose(env, "can't modify return codes of BPF programs\n");
+				ret = -EINVAL;
+				goto out;
+			}
 			ret = check_attach_modify_return(prog, addr);
 			if (ret)
 				verbose(env, "%s() is not modifiable\n",