From: Daniel Vetter <daniel@ffwll.ch>
To: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Cc: tzimmermann@suse.de, airlied@linux.ie,
gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org,
dri-devel@lists.freedesktop.org, skhan@linuxfoundation.org,
linux-kernel-mentees@lists.linuxfoundation.org,
Dan Carpenter <dan.carpenter@oracle.com>
Subject: Re: [PATCH] drm: Lock pointer access in drm_master_release()
Date: Thu, 10 Jun 2021 12:10:53 +0200 [thread overview]
Message-ID: <YMHlLQHRLWgWlXTs@phenom.ffwll.local> (raw)
In-Reply-To: <20210609092119.173590-1-desmondcheongzx@gmail.com>
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@oracle.com>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.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.
Are you up to do that fix too?
I think the drm_lease.c code also needs an audit, there we'd need to make
sure that we hold hold either the lock or a full master reference to avoid
the use-after-free issues here.
Patch merged to drm-misc-fixes with cc: stable.
-Daniel
> ---
> drivers/gpu/drm/drm_auth.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> index f00e5abdbbf4..b59b26a71ad5 100644
> --- a/drivers/gpu/drm/drm_auth.c
> +++ b/drivers/gpu/drm/drm_auth.c
> @@ -315,9 +315,10 @@ int drm_master_open(struct drm_file *file_priv)
> void drm_master_release(struct drm_file *file_priv)
> {
> struct drm_device *dev = file_priv->minor->dev;
> - struct drm_master *master = file_priv->master;
> + struct drm_master *master;
>
> mutex_lock(&dev->master_mutex);
> + master = file_priv->master;
> if (file_priv->magic)
> idr_remove(&file_priv->master->magic_map, file_priv->magic);
>
> --
> 2.25.1
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
next prev parent reply other threads:[~2021-06-10 10:10 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-09 9:21 [PATCH] drm: Lock pointer access in drm_master_release() Desmond Cheong Zhi Xi
2021-06-10 10:10 ` Daniel Vetter [this message]
2021-06-10 15:21 ` Desmond Cheong Zhi Xi
2021-06-10 16:48 ` Daniel Vetter
2021-06-11 2:18 ` Desmond Cheong Zhi Xi
2021-06-11 7:26 ` Daniel Vetter
2021-06-10 17:49 ` Emil Velikov
2021-06-11 3:10 ` Desmond Cheong Zhi Xi
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