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

From: Kees Cook <keescook@chromium.org>
To: Petr Mladek <pmladek@suse.com>
Cc: Kees Cook <keescook@chromium.org>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	John Ogness <john.ogness@linutronix.de>,
	Vijay Balakrishna <vijayb@linux.microsoft.com>,
	stable@vger.kernel.org, Tony Luck <tony.luck@intel.com>,
	"Guilherme G. Piccoli" <gpiccoli@igalia.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: [PATCH] printk: ringbuffer: Fix truncating buffer size min_t cast
Date: Thu, 10 Aug 2023 22:45:32 -0700	[thread overview]
Message-ID: <20230811054528.never.165-kees@kernel.org> (raw)

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@suse.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: John Ogness <john.ogness@linutronix.de>
Reported-by: Vijay Balakrishna <vijayb@linux.microsoft.com>
Closes: https://lore.kernel.org/lkml/d8bb1ec7-a4c5-43a2-9de0-9643a70b899f@linux.microsoft.com/
Fixes: b6cf8b3f3312 ("printk: add lockless ringbuffer")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 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

