[PATCH 5.8 285/633] bpf: disallow attaching modify_return tracing functions to other BPF programs

Greg Kroah-Hartman
2020-10-27T15:11:30Z

From: Toke Høiland-Jørgensen <toke@redhat.com>

[ Upstream commit 1af9270e908cd50a4f5d815c9b6f794c7d57ed07 ]

>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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/bpf/verifier.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 89b07db146763..c953dfbbaa6a9 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10862,6 +10862,11 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
}

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",
--
2.25.1