[PATCH 5.7 31/79] bpf: Fix NULL pointer dereference in __btf_resolve_helper_id()

[Greg Kroah-Hartman]

From: Peilin Ye <yepeilin.cs@xxxxxxxxx>

[ Upstream commit 5b801dfb7feb2738975d80223efc2fc193e55573 ]

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

    https://syzkaller.appspot.com/bug?id=f823224ada908fa5c207902a5a62065e53ca0fcc

Reported-by: syzbot+ee09bda7017345f1fbe6@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Peilin Ye <yepeilin.cs@xxxxxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/bpf/20200714180904.277512-1-yepeilin.cs@xxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 kernel/bpf/btf.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index d1f5d428c9fe2..6cafc596631c3 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -4011,6 +4011,11 @@ 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