[PATCH] drm: Lock pointer access in drm_master_release()

Daniel Vetter daniel at ffwll.ch
Thu Jun 10 10:10:53 UTC 2021

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 at oracle.com>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx at 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.

> ---
>  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

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