[PATCH 4.9 170/245] seq_buf: Fix overflow in seq_buf_putmem_hex()

[Greg Kroah-Hartman]

From: Yun Zhou <yun.zhou@xxxxxxxxxxxxx>

commit d3b16034a24a112bb83aeb669ac5b9b01f744bb7 upstream.

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.

Link: https://lore.kernel.org/lkml/20210625122453.5e2fe304@xxxxxxxxxxxxxxxx/
Link: https://lkml.kernel.org/r/20210626032156.47889-1-yun.zhou@xxxxxxxxxxxxx

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 5e3ca0ec76fce ("ftrace: introduce the "hex" output method")
Signed-off-by: Yun Zhou <yun.zhou@xxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 lib/seq_buf.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/lib/seq_buf.c
+++ b/lib/seq_buf.c
@@ -227,8 +227,10 @@ int seq_buf_putmem_hex(struct seq_buf *s
 
 	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