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From:   Daniel Vetter <daniel@ffwll.ch>
Date:   Wed, 21 Jul 2021 15:23:52 +0200
Message-ID: <CAKMK7uF4Y+WbiXWyvn1em4=xZAFvBusRhxYg9ue5TE003tovbA@mail.gmail.com>
Subject: Re: [PATCH v8 0/5] drm: address potential UAF bugs with drm_master ptrs
To:     Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Cc:     Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Dave Airlie <airlied@linux.ie>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        =?UTF-8?Q?Christian_K=C3=B6nig?= <christian.koenig@amd.com>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        intel-gfx <intel-gfx@lists.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@vger.kernel.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@lists.linaro.org>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        linux-kernel-mentees@lists.linuxfoundation.org,
        Emil Velikov <emil.l.velikov@gmail.com>
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On Wed, Jul 21, 2021 at 2:44 PM Desmond Cheong Zhi Xi
<desmondcheongzx@gmail.com> wrote:
> On 21/7/21 6:29 pm, Daniel Vetter wrote:
> > On Wed, Jul 21, 2021 at 6:12 AM Desmond Cheong Zhi Xi
> > <desmondcheongzx@gmail.com> wrote:
> >> On 21/7/21 2:24 am, Daniel Vetter wrote:
> >>> On Mon, Jul 12, 2021 at 12:35:03PM +0800, Desmond Cheong Zhi Xi wrote=
:
> >>>> Hi,
> >>>>
> >>>> In the previous thread on this series we decided to remove a patch t=
hat was violating a lockdep requirement in drm_lease. In addition to this c=
hange, I took a closer look at the CI logs for the Basic Acceptance Tests a=
nd noticed that another regression was introduced. The new patch 2 is a res=
ponse to this.
> >>>>
> >>>> Overall, this series addresses potential use-after-free errors when =
dereferencing pointers to struct drm_master. These were identified after on=
e such bug was caught by Syzbot in drm_getunique():
> >>>> https://syzkaller.appspot.com/bug?id=3D148d2f1dfac64af52ffd27b661981=
a540724f803
> >>>>
> >>>> The series is broken up into five patches:
> >>>>
> >>>> 1. Move a call to drm_is_current_master() out from a section locked =
by &dev->mode_config.mutex in drm_mode_getconnector(). This patch does not =
apply to stable.
> >>>>
> >>>> 2. Move a call to drm_is_current_master() out from the RCU read-side=
 critical section in drm_clients_info().
> >>>>
> >>>> 3. Implement a locked version of drm_is_current_master() function th=
at's used within drm_auth.c.
> >>>>
> >>>> 4. Serialize drm_file.master by introducing a new spinlock that's he=
ld whenever the value of drm_file.master changes.
> >>>>
> >>>> 5. Identify areas in drm_lease.c where pointers to struct drm_master=
 are dereferenced, and ensure that the master pointers are not freed during=
 use.
> >>>>
> >>>> v7 -> v8:
> >>>> - Remove the patch that moves the call to _drm_lease_held out from t=
he section locked by &dev->mode_config.idr_mutex in __drm_mode_object_find.=
 This patch violated an existing lockdep requirement as reported by the int=
el-gfx CI.
> >>>> - Added a new patch that moves a call to drm_is_current_master out f=
rom the RCU critical section in drm_clients_info. This was reported by the =
intel-gfx CI.
> >>>>
> >>>> v6 -> v7:
> >>>> - Modify code alignment as suggested by the intel-gfx CI.
> >>>> - Add a new patch to the series that adds a new lock to serialize dr=
m_file.master, in response to the lockdep splat by the intel-gfx CI.
> >>>> - Update drm_file_get_master to use the new drm_file.master_lock ins=
tead of drm_device.master_mutex, in response to the lockdep splat by the in=
tel-gfx CI.
> >>>>
> >>>> v5 -> v6:
> >>>> - Add a new patch to the series that moves the call to _drm_lease_he=
ld out from the section locked by &dev->mode_config.idr_mutex in __drm_mode=
_object_find.
> >>>> - Clarify the kerneldoc for dereferencing drm_file.master, as sugges=
ted by Daniel Vetter.
> >>>> - Refactor error paths with goto labels so that each function only h=
as a single drm_master_put(), as suggested by Emil Velikov.
> >>>> - Modify comparisons to NULL into "!master", as suggested by the int=
el-gfx CI.
> >>>>
> >>>> v4 -> v5:
> >>>> - Add a new patch to the series that moves the call to drm_is_curren=
t_master in drm_mode_getconnector out from the section locked by &dev->mode=
_config.mutex.
> >>>> - Additionally, added a missing semicolon to the patch, caught by th=
e intel-gfx CI.
> >>>>
> >>>> v3 -> v4:
> >>>> - Move the call to drm_is_current_master in drm_mode_getconnector ou=
t from the section locked by &dev->mode_config.mutex. As suggested by Danie=
l Vetter. This avoids a circular lock lock dependency as reported here http=
s://patchwork.freedesktop.org/patch/440406/
> >>>> - Inside drm_is_current_master, instead of grabbing &fpriv->master->=
dev->master_mutex, we grab &fpriv->minor->dev->master_mutex to avoid derefe=
rencing a null ptr if fpriv->master is not set.
> >>>> - Modify kerneldoc formatting for drm_file.master, as suggested by D=
aniel Vetter.
> >>>> - Additionally, add a file_priv->master NULL check inside drm_file_g=
et_master, and handle the NULL result accordingly in drm_lease.c. As sugges=
ted by Daniel Vetter.
> >>>>
> >>>> v2 -> v3:
> >>>> - Move the definition of drm_is_current_master and the _locked versi=
on higher up in drm_auth.c to avoid needing a forward declaration of drm_is=
_current_master_locked. As suggested by Daniel Vetter.
> >>>> - Instead of leaking drm_device.master_mutex into drm_lease.c to pro=
tect drm_master pointers, add a new drm_file_get_master() function that ret=
urns drm_file->master while increasing its reference count, to prevent drm_=
file->master from being freed. As suggested by Daniel Vetter.
> >>>>
> >>>> v1 -> v2:
> >>>> - Move the lock and assignment before the DRM_DEBUG_LEASE in drm_mod=
e_get_lease_ioctl, as suggested by Emil Velikov.
> >>>
> >>> Apologies for the delay, I missed your series. Maybe just ping next t=
ime
> >>> around there's silence.
> >>>
> >>> Looks all great, merged to drm-misc-next. Given how complex this was =
I'm
> >>> vary of just pushing this to -fixes without some solid testing.
> >>>
> >>
> >> Hi Daniel,
> >>
> >> Thanks for merging, more testing definitely sounds good to me.
> >>
> >>> One thing I noticed is that drm_is_current_master could just use the
> >>> spinlock, since it's only doing a read access. Care to type up that p=
atch?
> >>>
> >>
> >> I thought about this too, but I'm not sure if that's the best solution=
.
> >>
> >> drm_is_current_master calls drm_lease_owner which then walks up the tr=
ee
> >> of master lessors. The spinlock protects the master of the current drm
> >> file, but subsequent lessors aren't protected without holding the
> >> device's master mutex.
> >
> > But this isn't a fpriv->master pointer, but a master->lessor pointer.
> > Which should never ever be able to change (we'd have tons of uaf bugs
> > around drm_lease_owner otherwise). So I don't think there's anything
> > that dev->master_lock protects here that fpriv->master_lookup_lock
> > doesn't protect already?
> >
> > Or am I missing something?
> >  > The comment in the struct drm_master says it's protected by
> > mode_config.idr_mutex, but that only applies to the idrs and lists I
> > think.
> >
>
> Ah you're right, I also completely forgot that lessees hold a reference
> to their lessor so nothing will be freed as long as the spinlock is
> held. I'll prepare that patch then, thanks for pointing it out.

btw since we now looked at all this in detail, can you perhaps do a
patch to update the kerneldoc for all the lease fields in struct
drm_master? I think moving them to the inline style and then adding
comments for each field how locking/lifetime rules work would be
really good. Since right now it's all fresh from for us.
-Daniel

> >>> Also, do you plan to look into that idea we've discussed to flush pen=
ding
> >>> access when we revoke a master or a lease? I think that would be real=
ly
> >>> nice improvement here.
> >>> -Daniel
> >>>
> >>
> >> Yup, now that the potential UAFs are addressed (hopefully), I'll take =
a
> >> closer look and propose a patch for this.
> >
> > Thanks a lot.
> > -Daniel
> >
> >>
> >> Best wishes,
> >> Desmond
> >>
> >>>>
> >>>> Desmond Cheong Zhi Xi (5):
> >>>>     drm: avoid circular locks in drm_mode_getconnector
> >>>>     drm: avoid blocking in drm_clients_info's rcu section
> >>>>     drm: add a locked version of drm_is_current_master
> >>>>     drm: serialize drm_file.master with a new spinlock
> >>>>     drm: protect drm_master pointers in drm_lease.c
> >>>>
> >>>>    drivers/gpu/drm/drm_auth.c      | 93 ++++++++++++++++++++++++----=
-----
> >>>>    drivers/gpu/drm/drm_connector.c |  5 +-
> >>>>    drivers/gpu/drm/drm_debugfs.c   |  3 +-
> >>>>    drivers/gpu/drm/drm_file.c      |  1 +
> >>>>    drivers/gpu/drm/drm_lease.c     | 81 +++++++++++++++++++++-------
> >>>>    include/drm/drm_auth.h          |  1 +
> >>>>    include/drm/drm_file.h          | 18 +++++--
> >>>>    7 files changed, 152 insertions(+), 50 deletions(-)
> >>>>
> >>>> --
> >>>> 2.25.1
> >>>>
> >>>
> >>
> >
> >
>


--=20
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch