linux-kernel - Re: [PATCH] udf: udftime: prevent overflow in udf_disk_stamp_to

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Message-ID: <20240322134855.qm2pzjvvagufgrbk@quack3>
Date: Fri, 22 Mar 2024 14:48:55 +0100
From: Jan Kara <jack@...e.cz>
To: Roman Smirnov <r.smirnov@....ru>
Cc: Jan Kara <jack@...e.com>, linux-kernel@...r.kernel.org,
	Sergey Shtylyov <s.shtylyov@....ru>,
	Karina Yankevich <k.yankevich@....ru>, lvc-project@...uxtesting.org
Subject: Re: [PATCH] udf: udftime: prevent overflow in
 udf_disk_stamp_to_time()

On Fri 22-03-24 12:01:45, Roman Smirnov wrote:
> An overflow can occur in a situation where src.centiseconds
> takes the value of 255. This situation is unlikely, but there
> is no validation check anywere in the code. It is necessary
> to convert the type of expression to 64-bit.
> 
> Found by Linux Verification Center (linuxtesting.org) with Svace.
> 
> Suggested-by: Sergey Shtylyov <s.shtylyov@....ru>
> Signed-off-by: Roman Smirnov <r.smirnov@....ru>
> Reviewed-by: Sergey Shtylyov <s.shtylyov@....ru>

Thanks for the fix but I think this is not the right way to solve the
problem. Mainly because the 64-bit division is unnecessarily costly. I'd
rather first make sure that src.centiseconds < 100,
src.hundredsOfMicroseconds < 100, src.microseconds < 100 (just ignore the
values if any is bogus) and then do the multiplication without worrying it
might overflow the int type...

								Honza

> @@ -46,13 +47,13 @@ udf_disk_stamp_to_time(struct timespec64 *dest, struct timestamp src)
>  	dest->tv_sec = mktime64(year, src.month, src.day, src.hour, src.minute,
>  			src.second);
>  	dest->tv_sec -= offset * 60;
> -	dest->tv_nsec = 1000 * (src.centiseconds * 10000 +
> +	nsec = 1000LL * (src.centiseconds * 10000 +
>  			src.hundredsOfMicroseconds * 100 + src.microseconds);
>  	/*
>  	 * Sanitize nanosecond field since reportedly some filesystems are
>  	 * recorded with bogus sub-second values.
>  	 */
> -	dest->tv_nsec %= NSEC_PER_SEC;
> +	dest->tv_nsec = do_div(nsec, NSEC_PER_SEC);
>  }
>  
>  void
> -- 
> 2.34.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR