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

[Tyler Hicks <code@tyhicks.com>]

On 2023-08-10 22:45:32, Kees Cook wrote:
> 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>

Nice find!

Reviewed-by: Tyler Hicks (Microsoft) <code@tyhicks.com>
Tested-by: Tyler Hicks (Microsoft) <code@tyhicks.com>

Verified the fix by applying it to an instrumented v6.5-rc5 kernel that
allows userspace to execute kmsg_dump(), detects NULL bytes in data
copied from the ring buffer, and warns about invalid truncation due to
the min_t(u16, ...) casting bug. Everything looks good!

Tyler

> ---
>  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
>