linux-hardening - Re: [PATCH] libfs: fix accidental overflow in offset calculation

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Date: Fri, 10 May 2024 05:48:05 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Nathan Chancellor <nathan@...nel.org>,
	Bill Wendling <morbo@...gle.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] libfs: fix accidental overflow in offset calculation

On Fri, May 10, 2024 at 03:26:08AM +0000, Justin Stitt wrote:

> This feels like a case of accidental correctness. You demonstrated that
> even with overflow we end up going down a control path that returns an
> error code so all is good.

No.  It's about a very simple arithmetical fact: the smallest number that
wraps to 0 is 2^N, which is more than twice the maximal signed N-bit
value.  So wraparound on adding a signed N-bit to non-negative signed N-bit
will always end up with negative result.  That's *NOT* a hard math.  Really.

As for the rest... SEEK_CUR semantics is "seek to current position + offset";
just about any ->llseek() instance will have that shape - calculate the
position we want to get to, then forget about the difference between
SEEK_SET and SEEK_CUR.  So noticing that wraparound ends with negative
is enough - we reject straight SEEK_SET to negatives anyway, so no
extra logics is needed.

> However, I think finding the solution
> shouldn't require as much mental gymnastics. We clearly don't want our
> file offsets to wraparound and a plain-and-simple check for that lets
> readers of the code understand this.

No comments that would be suitable for any kind of polite company.