linux-kernel - Re: [PATCH] fs/read_write.c: Fix a broken signed integer overflow check.

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Message-ID: <YgEzs2Hp0LrdDmJu@zeniv-ca.linux.org.uk>
Date:   Mon, 7 Feb 2022 14:58:59 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Ari Sundholm <ari@...era.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        stable@...r.kernel.org, Anton Altaparmakov <anton@...era.com>
Subject: Re: [PATCH] fs/read_write.c: Fix a broken signed integer overflow
 check.

On Mon, Feb 07, 2022 at 02:07:11PM +0200, Ari Sundholm wrote:
> The function generic_copy_file_checks() checks that the ends of the
> input and output file ranges do not overflow. Unfortunately, there is
> an issue with the check itself.
> 
> Due to the integer promotion rules in C, the expressions
> (pos_in + count) and (pos_out + count) have an unsigned type because
> the count variable has the type uint64_t. Thus, in many cases where we
> should detect signed integer overflow to have occurred (and thus one or
> more of the ranges being invalid), the expressions will instead be
> interpreted as large unsigned integers. This means the check is broken.

I must be slow this morning, but... which values of pos_in and count are
caught by your check, but not by the original?

> -	if (pos_in + count < pos_in || pos_out + count < pos_out)
> +	if ((loff_t)(pos_in + count) < pos_in ||
> +			(loff_t)(pos_out + count) < pos_out)

Example, please.  Why do you need that comparison to be signed?