From: Wenwen Wang <wang6495@umn.edu>
Date: Sun, 7 Oct 2018 15:23:15 -0500
Subject: bpf: btf: Fix a missing check bug
Patch-mainline: v4.20-rc1
Git-commit: 8af03d1ae2e154a8be3631e8694b87007e1bdbc2
References: bsc#1109837

In btf_parse_hdr(), the length of the btf data header is firstly copied
from the user space to 'hdr_len' and checked to see whether it is larger
than 'btf_data_size'. If yes, an error code EINVAL is returned. Otherwise,
the whole header is copied again from the user space to 'btf->hdr'.
However, after the second copy, there is no check between
'btf->hdr->hdr_len' and 'hdr_len' to confirm that the two copies get the
same value. Given that the btf data is in the user space, a malicious user
can race to change the data between the two copies. By doing so, the user
can provide malicious data to the kernel and cause undefined behavior.

This patch adds a necessary check after the second copy, to make sure
'btf->hdr->hdr_len' has the same value as 'hdr_len'. Otherwise, an error
code EINVAL will be returned.

Signed-off-by: Wenwen Wang <wang6495@umn.edu>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 kernel/bpf/btf.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -2114,6 +2114,9 @@ static int btf_parse_hdr(struct btf_veri
 
 	hdr = &btf->hdr;
 
+	if (hdr->hdr_len != hdr_len)
+		return -EINVAL;
+
 	btf_verifier_log_hdr(env, btf_data_size);
 
 	if (hdr->magic != BTF_MAGIC) {