linux-kernel - Re: [PATCH] drm: Lock pointer access in drm_master

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Date:   Thu, 10 Jun 2021 18:49:09 +0100
From:   Emil Velikov <emil.l.velikov@...il.com>
To:     Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Dave Airlie <airlied@...ux.ie>,
        ML dri-devel <dri-devel@...ts.freedesktop.org>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
        skhan@...uxfoundation.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [PATCH] drm: Lock pointer access in drm_master_release()

On Thu, 10 Jun 2021 at 11:10, Daniel Vetter <daniel@...ll.ch> wrote:
>
> On Wed, Jun 09, 2021 at 05:21:19PM +0800, Desmond Cheong Zhi Xi wrote:
> > This patch eliminates the following smatch warning:
> > drivers/gpu/drm/drm_auth.c:320 drm_master_release() warn: unlocked access 'master' (line 318) expected lock '&dev->master_mutex'
> >
> > The 'file_priv->master' field should be protected by the mutex lock to
> > '&dev->master_mutex'. This is because other processes can concurrently
> > modify this field and free the current 'file_priv->master'
> > pointer. This could result in a use-after-free error when 'master' is
> > dereferenced in subsequent function calls to
> > 'drm_legacy_lock_master_cleanup()' or to 'drm_lease_revoke()'.
> >
> > An example of a scenario that would produce this error can be seen
> > from a similar bug in 'drm_getunique()' that was reported by Syzbot:
> > https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f803
> >
> > In the Syzbot report, another process concurrently acquired the
> > device's master mutex in 'drm_setmaster_ioctl()', then overwrote
> > 'fpriv->master' in 'drm_new_set_master()'. The old value of
> > 'fpriv->master' was subsequently freed before the mutex was unlocked.
> >
> > Reported-by: Dan Carpenter <dan.carpenter@...cle.com>
> > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
>
> Thanks a lot. I've done an audit of this code, and I found another
> potential problem in drm_is_current_master. The callers from drm_auth.c
> hold the dev->master_mutex, but all the external ones dont. I think we
> need to split this into a _locked function for use within drm_auth.c, and
> the exported one needs to grab the dev->master_mutex while it's checking
> master status. Ofc there will still be races, those are ok, but right now
> we run the risk of use-after free problems in drm_lease_owner.
>
Note that some code does acquire the mutex via
drm_master_internal_acquire - so we should be careful.
As mentioned elsewhere - having a _locked version of
drm_is_current_master sounds good.

Might as well throw a lockdep_assert_held_once in there just in case :-P

Happy to help review the follow-up patches.
-Emil