[PATCH 6.5 646/739] printk: ringbuffer: Fix truncating buffer size min_t cast

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6.5-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Kees Cook <keescook@xxxxxxxxxxxx>

commit 53e9e33ede37a247d926db5e4a9e56b55204e66c upstream.

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>
Link: 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>
Tested-by: Vijay Balakrishna <vijayb@xxxxxxxxxxxxxxxxxxx>
Tested-by: Guilherme G. Piccoli <gpiccoli@xxxxxxxxxx> # Steam Deck
Reviewed-by: Tyler Hicks (Microsoft) <code@xxxxxxxxxxx>
Tested-by: Tyler Hicks (Microsoft) <code@xxxxxxxxxxx>
Reviewed-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Reviewed-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
Link: https://lore.kernel.org/r/20230811054528.never.165-kees@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/printk/printk_ringbuffer.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -1735,7 +1735,7 @@ static bool copy_data(struct prb_data_ri
 	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;





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