seq_buf: Fix overflow in seq_buf_putmem_hex()

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@oasis.local.home/
Link: https://lkml.kernel.org/r/20210626032156.47889-1-yun.zhou@windriver.com
Cc: stable@vger.kernel.org
Fixes: 5e3ca0ec76fce ("ftrace: introduce the "hex" output method")
Signed-off-by: Yun Zhou <yun.zhou@windriver.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/lib/seq_buf.c b/lib/seq_buf.c
index 707453f..62c2059 100644 (file)
--- a/lib/seq_buf.c
+++ b/lib/seq_buf.c
@@ -229,8 +229,10 @@ 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