linux-kernel - Re: [RFC 2/3] blktrace: fix debugfs use after free

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Date:   Thu, 2 Apr 2020 16:14:04 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Eric Sandeen <sandeen@...deen.net>
Cc:     axboe@...nel.dk, viro@...iv.linux.org.uk,
        gregkh@...uxfoundation.org, rostedt@...dmis.org, mingo@...hat.com,
        jack@...e.cz, ming.lei@...hat.com, nstange@...e.de,
        mhocko@...e.com, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Bart Van Assche <bvanassche@....org>,
        Omar Sandoval <osandov@...com>,
        Hannes Reinecke <hare@...e.com>,
        Michal Hocko <mhocko@...nel.org>,
        syzbot+603294af2d01acfdd6da@...kaller.appspotmail.com
Subject: Re: [RFC 2/3] blktrace: fix debugfs use after free

On Wed, Apr 01, 2020 at 08:57:42PM -0500, Eric Sandeen wrote:
> On 4/1/20 7:00 PM, Luis Chamberlain wrote:
> > On commit 6ac93117ab00 ("blktrace: use existing disk debugfs directory")
> > Omar fixed the original blktrace code for multiqueue use. This however
> > left in place a possible crash, if you happen to abuse blktrace in a way
> > it was not intended.
> > 
> > Namely, if you loop adding a device, setup the blktrace with BLKTRACESETUP,
> > forget to BLKTRACETEARDOWN, and then just remove the device you end up
> > with a panic:
> 
> Weird, I swear I tested that and didn't hit it, but ...

The real issue was also the dangling block device, a loop device proves
easy to test that. I'll note that another way to test this as well would
be to have a virtual machine with a block devices that goes in and out
via whatever virsh shenanigans to make a block device appear / disappear.

> > This issue can be reproduced with break-blktrace [2] using:
> > 
> >   break-blktrace -c 10 -d
> 
> + -s, right?

Yeap, sorry about that.

> > This patch fixes this issue. Note that there is also another
> > respective UAF but from the ioctl path [3], this should also fix
> > that issue.
> > 
> > This patch then also contends the severity of CVE-2019-19770 as
> > this issue is only possible using root to shoot yourself in the
> > foot by also misuing blktrace.
> > 
> > [0] https://bugzilla.kernel.org/show_bug.cgi?id=205713
> > [1] https://nvd.nist.gov/vuln/detail/CVE-2019-19770
> > [2] https://github.com/mcgrof/break-blktrace
> 
> I verified that this does reproduce the exact same KASAN splat on
> kernel 4.19.83 as reported in the original bug, thanks!

I just codified what Nicolai suspected, we should thank him :)

  Luis