linux-kernel - [Linux-kernel-mentees] [PATCH v2] bpf: Fix NULL pointer dereference in __btf_resolve_helper

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Date:   Mon, 13 Jul 2020 21:27:32 -0400
From:   Peilin Ye <yepeilin.cs@...il.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     Peilin Ye <yepeilin.cs@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        syzkaller-bugs@...glegroups.com,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: [Linux-kernel-mentees] [PATCH v2] bpf: Fix NULL pointer dereference in __btf_resolve_helper_id()

Prevent __btf_resolve_helper_id() from dereferencing `btf_vmlinux`
as NULL. This patch fixes the following syzbot bug:

    https://syzkaller.appspot.com/bug?id=5edd146856fd513747c1992442732e5a0e9ba355

Reported-by: syzbot+ee09bda7017345f1fbe6@...kaller.appspotmail.com
Signed-off-by: Peilin Ye <yepeilin.cs@...il.com>
---
Thank you for reviewing my patch! I am new to Linux kernel development; would
the log message and errno be appropriate for this case?

Change in v2:
    - Split NULL and IS_ERR cases.

 kernel/bpf/btf.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 30721f2c2d10..092116a311f4 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -4088,6 +4088,11 @@ static int __btf_resolve_helper_id(struct bpf_verifier_log *log, void *fn,
 	const char *tname, *sym;
 	u32 btf_id, i;
 
+	if (!btf_vmlinux) {
+		bpf_log(log, "btf_vmlinux doesn't exist\n");
+		return -EINVAL;
+	}
+
 	if (IS_ERR(btf_vmlinux)) {
 		bpf_log(log, "btf_vmlinux is malformed\n");
 		return -EINVAL;
-- 
2.25.1