linux-kernel - Re: [PATCH] fs/aio: fix uaf in sys_io

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Date: Mon, 4 Mar 2024 12:47:21 -0500
From: Benjamin LaHaise <ben@...munityfibre.ca>
To: Bart Van Assche <bvanassche@....org>
Cc: Edward Adam Davis <eadavis@...com>,
	syzbot+b91eb2ed18f599dd3c31@...kaller.appspotmail.com,
	brauner@...nel.org, jack@...e.cz, linux-aio@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk
Subject: Re: [PATCH] fs/aio: fix uaf in sys_io_cancel

On Mon, Mar 04, 2024 at 09:40:35AM -0800, Bart Van Assche wrote:
> On 3/4/24 09:31, Benjamin LaHaise wrote:
> >A revert is justified when a series of patches is buggy and had
> >insufficient review prior to merging.
> 
> That's not how Linux kernel development works. If a bug can get fixed
> easily, a fix is preferred instead of reverting + reapplying a patch.

Your original "fix" is not right, and it wasn't properly tested.  Commit
54cbc058d86beca3515c994039b5c0f0a34f53dd needs to be reverted.

> >Using the "a kernel warning hit" approach for work on cancellation is
> >very much a sign that the patches were half baked.
> Is there perhaps a misunderstanding? My patches fix a kernel warning and
> did not introduce any new WARN*() statements.

The change that introduced that callback by you was incorrect and should
be reverted.

> >Why are you touching the kiocb after ownership has already been
> >passed on to another entity?
> Touching the kiocb after ownership has been passed is the result of an
> oversight. Whether or not kiocb->ki_cancel() transfers ownership depends
> on the I/O type. The use-after-free was not introduced on purpose.

Your fix is still incorrect.  You're still touching memory that you don't
own.  The event should be generated via the ->ki_cancel method, not in the
io_cancel() syscall.

		-ben

> Bart.
> 
> 

-- 
"Thought is the essence of where you are now."