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From:   Wenwen Wang <wang6495@umn.edu>
To:     Wenwen Wang <wang6495@umn.edu>
Cc:     Kangjie Lu <kjlu@umn.edu>, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        netdev@vger.kernel.org (open list:BPF (Safe dynamic programs and tools)),
        linux-kernel@vger.kernel.org (open list:BPF (Safe dynamic programs and
        tools))
Subject: [PATCH] bpf: btf: Fix a missing check bug
Date:   Sun,  7 Oct 2018 15:23:15 -0500
Message-Id: <1538943795-30895-1-git-send-email-wang6495@umn.edu>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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>
---
 kernel/bpf/btf.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 2590700..7cce7db 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -2114,6 +2114,9 @@ static int btf_parse_hdr(struct btf_verifier_env *env, void __user *btf_data,
 
 	hdr = &btf->hdr;
 
+	if (hdr->hdr_len != hdr_len)
+		return -EINVAL;
+
 	btf_verifier_log_hdr(env, btf_data_size);
 
 	if (hdr->magic != BTF_MAGIC) {
-- 
2.7.4