linux-hardening - Re: [PATCH v3] fs: fix unintentional arithmetic wraparound in offset calculation

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Message-ID: <CAFhGd8pQKE_uak2gqUXjbQy4LCGoJqVD2XCZrOC606u-tzS0mg@mail.gmail.com>
Date: Fri, 17 May 2024 13:40:59 -0700
From: Justin Stitt <justinstitt@...gle.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Nick Desaulniers <ndesaulniers@...gle.com>, Nathan Chancellor <nathan@...nel.org>, 
	Alexander Viro <viro@...iv.linux.org.uk>, 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 v3] fs: fix unintentional arithmetic wraparound in offset calculation

Hi,

On Thu, May 16, 2024 at 6:13 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Fri, May 17, 2024 at 12:29:06AM +0000, Justin Stitt wrote:
> > When running syzkaller with the newly reintroduced signed integer
> > overflow sanitizer we encounter this report:
>
> why do you keep saying it's unintentional?  it's clearly intended.

Right, "unintentional" is a poor choice of phrasing. I actually mean:
"overflow-checking arithmetic was done in a way that intrinsically
causes an overflow (wraparound)".

I can clearly see the intent of the code; there's even comments saying
exactly what it does: "/* Ensure offsets don't wrap. */"... So the
thinking is: let's use the overflow-checking helpers so we can get a
good signal through the sanitizers on _real_ bugs, especially in spots
with no bounds handling.

Thanks
Justin