[PATCH] printk: ringbuffer: Fix truncating buffer size min_t cast

[Kees Cook]

If an output buffer size exceeded U16_MAX, the min_t(u16, ...) cast in
copy_data() was causing writes to truncate. This manifested as output
bytes being skipped, seen as %NUL bytes in pstore dumps when the available
record size was larger than 65536. Fix the cast to no longer truncate
the calculation.

Cc: Petr Mladek <pmladek@xxxxxxxx>
Cc: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: John Ogness <john.ogness@xxxxxxxxxxxxx>
Reported-by: Vijay Balakrishna <vijayb@xxxxxxxxxxxxxxxxxxx>
Closes: https://lore.kernel.org/lkml/d8bb1ec7-a4c5-43a2-9de0-9643a70b899f@xxxxxxxxxxxxxxxxxxx/
Fixes: b6cf8b3f3312 ("printk: add lockless ringbuffer")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 kernel/printk/printk_ringbuffer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index 2dc4d5a1f1ff..fde338606ce8 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -1735,7 +1735,7 @@ static bool copy_data(struct prb_data_ring *data_ring,
 	if (!buf || !buf_size)
 		return true;
 
-	data_size = min_t(u16, buf_size, len);
+	data_size = min_t(unsigned int, buf_size, len);
 
 	memcpy(&buf[0], data, data_size); /* LMM(copy_data:A) */
 	return true;
-- 
2.34.1