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

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Message-ID: <202405171417.B290C50A@keescook>
Date: Fri, 17 May 2024 14:24:17 -0700
From: Kees Cook <keescook@...omium.org>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Matthew Wilcox <willy@...radead.org>,
	Justin Stitt <justinstitt@...gle.com>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	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 v3] fs: fix unintentional arithmetic wraparound in offset
 calculation

On Fri, May 17, 2024 at 02:26:47AM +0100, Al Viro wrote:
> On Fri, May 17, 2024 at 02:13:22AM +0100, Matthew Wilcox 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.
> 
> Because they are short on actual bugs to be found by their tooling
> and attempt to inflate the sound/noise rate; therefore, every time

"short on bugs"? We're trying to drive it to zero. I would *love* to be
short on bugs. See my reply[1] to Ted.

> when overflow _IS_ handled correctly, it must have been an accident -
> we couldn't have possibly done the analysis correctly.  And if somebody
> insists that they _are_ capable of basic math, they must be dishonest.
> So... "unintentional" it's going to be.

As Justin said, this is a poor choice in wording. In other cases I've
tried to describe this as making changes so that intent is unambiguous
(to both a human and a compiler).

> <southpark> Math is hard, mmkay?  </southpark>
> 
> Al, more than slightly annoyed by that aspect of the entire thing...

I'm sorry about that. None of this is a commentary on code correctness;
we're just trying to refactor things so that the compiler can help us
catch the _unintended_ overflows. This one is _intended_, so here we are
to find a palatable way to leave the behavior unchanged while gaining
compiler coverage.

-Kees

[1] https://lore.kernel.org/linux-hardening/202405171329.019F2F566C@keescook/

-- 
Kees Cook