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From:   Yun Zhou <yun.zhou@windriver.com>
To:     <rostedt@goodmis.org>
CC:     <linux-kernel@vger.kernel.org>,
        <kernel-hardening@lists.openwall.com>, <ying.xue@windriver.com>,
        <zhiquan.li@windriver.com>
Subject: [PATCH 1/2] seq_buf: fix overflow in seq_buf_putmem_hex()
Date:   Sat, 26 Jun 2021 11:21:55 +0800
Message-ID: <20210626032156.47889-1-yun.zhou@windriver.com>
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Precedence: bulk

There's two variables being increased in that loop (i and j), and i
follows the raw data, and j follows what is being written into the buffer.
We should compare 'i' to MAX_MEMHEX_BYTES or compare 'j' to HEX_CHARS.
Otherwise, if 'j' goes bigger than HEX_CHARS, it will overflow the
destination buffer.

This bug exists in the original code (commit 5e3ca0ec76fce 'ftrace:
introduce the "hex" output method'). Although its original design did
not support more than 8 bytes, the only check on length seems to have
mistaken the comparison object, 'len' should compare to 'HEX_CHARS/2'.
    BUG_ON(len >= HEX_CHARS);

Signed-off-by: Yun Zhou <yun.zhou@windriver.com>
---
 lib/seq_buf.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/seq_buf.c b/lib/seq_buf.c
index 6aabb609dd87..223fbc3bb958 100644
--- a/lib/seq_buf.c
+++ b/lib/seq_buf.c
@@ -228,8 +228,10 @@ int seq_buf_putmem_hex(struct seq_buf *s, const void *mem,
 
 	WARN_ON(s->size == 0);
 
+	BUILD_BUG_ON(MAX_MEMHEX_BYTES * 2 >= HEX_CHARS);
+
 	while (len) {
-		start_len = min(len, HEX_CHARS - 1);
+		start_len = min(len, MAX_MEMHEX_BYTES);
 #ifdef __BIG_ENDIAN
 		for (i = 0, j = 0; i < start_len; i++) {
 #else
-- 
2.26.1