On 18/6/21 1:12 am, Daniel Vetter wrote:
On Tue, Jun 15, 2021 at 10:36:45AM +0800, Desmond Cheong Zhi Xi wrote:
This patch ensures that the device's master mutex is acquired before
accessing pointers to struct drm_master that are subsequently
dereferenced. Without the mutex, the struct drm_master may be freed
concurrently by another process calling drm_setmaster_ioctl(). This
could then lead to use-after-free errors.
Reported-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
Reviewed-by: Emil Velikov <emil.l.velikov@xxxxxxxxx>
---
drivers/gpu/drm/drm_lease.c | 58 +++++++++++++++++++++++++++----------
1 file changed, 43 insertions(+), 15 deletions(-)
diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c
index da4f085fc09e..3e6f689236e5 100644
--- a/drivers/gpu/drm/drm_lease.c
+++ b/drivers/gpu/drm/drm_lease.c
@@ -107,10 +107,16 @@ static bool _drm_has_leased(struct drm_master *master, int id)
*/
bool _drm_lease_held(struct drm_file *file_priv, int id)
{
+ bool ret;
+
if (!file_priv || !file_priv->master)
return true;
- return _drm_lease_held_master(file_priv->master, id);
+ mutex_lock(&file_priv->master->dev->master_mutex);
So maybe we have a bug somewhere, and the kerneldoc isn't 100% clear, but
I thought file_priv->master is invariant over the lifetime of file_priv.
So we don't need a lock to check anything here.
It's the drm_device->master derefence that gets us into trouble. Well also
file_priv->is_owner is protected by dev->master_mutex.
So I think with your previous patch all the access here in drm_lease.c is
ok and already protected? Or am I missing something?
Thanks, Daniel
My thinking was that file_priv->master is invariant only if it is the
creator of master. If file_priv->is_master is false, then a call to
drm_setmaster_ioctl will invoke drm_new_set_master, which then allocates
a new master for file_priv, and puts the old master.
This could be an issue in _drm_lease_held_master, because we dereference
master to get master->dev, master->lessor, and master->leases.
With the same reasoning, in other parts of drm_lease.c, if there's an
access to drm_file->master that's subsequently dereferenced, I added a
lock around them.
I could definitely be mistaken on this, so apologies if this scenario
doesn't arise.
Best wishes,
Desmond
