Re: [PATCH v3 4/9] drm: fix potential null ptr dereferences in drm_{auth,ioctl}

On 18/8/21 6:11 pm, Daniel Vetter wrote:

On Wed, Aug 18, 2021 at 03:38:19PM +0800, Desmond Cheong Zhi Xi wrote:

quoted

There are three areas where we dereference struct drm_master without
checking if the pointer is non-NULL.

1. drm_getmagic is called from the ioctl_handler. Since
DRM_IOCTL_GET_MAGIC has no ioctl flags, drm_getmagic is run without
any check that drm_file.master has been set.

2. Similarly, drm_getunique is called from the ioctl_handler, but
DRM_IOCTL_GET_UNIQUE has no ioctl flags. So there is no guarantee that
drm_file.master has been set.

I think the above two are impossible, due to the refcounting rules for
struct file.

Right, will drop those two parts from the patch.

quoted

3. drm_master_release can also be called without having a
drm_file.master set. Here is one error path:
   drm_open():
     drm_open_helper():
       drm_master_open():
         drm_new_set_master(); <--- returns -ENOMEM,
                                    drm_file.master not set
       drm_file_free():
         drm_master_release(); <--- NULL ptr dereference
                                    (file_priv->master->magic_map)

Fix these by checking if the master pointers are NULL before use.

Signed-off-by: Desmond Cheong Zhi Xi <redacted>
---
  drivers/gpu/drm/drm_auth.c  | 16 ++++++++++++++--
  drivers/gpu/drm/drm_ioctl.c |  5 +++++
  2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index f9267b21556e..b7230604496b 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c

@@ -95,11 +95,18 @@ EXPORT_SYMBOL(drm_is_current_master);
  int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv)
  {
  	struct drm_auth *auth = data;
+	struct drm_master *master;
  	int ret = 0;
  
  	mutex_lock(&dev->master_mutex);
+	master = file_priv->master;
+	if (!master) {
+		mutex_unlock(&dev->master_mutex);
+		return -EINVAL;
+	}
+
  	if (!file_priv->magic) {
-		ret = idr_alloc(&file_priv->master->magic_map, file_priv,
+		ret = idr_alloc(&master->magic_map, file_priv,
  				1, 0, GFP_KERNEL);
  		if (ret >= 0)
  			file_priv->magic = ret;

@@ -355,8 +362,12 @@ void drm_master_release(struct drm_file *file_priv)
  
  	mutex_lock(&dev->master_mutex);
  	master = file_priv->master;
+
+	if (!master)
+		goto unlock;

This is a bit convoluted, since we're in the single-threaded release path
we don't need any locking for file_priv related things. Therefore we can
pull the master check out and just directly return.

But since it's a bit surprising maybe a comment that this can happen when
drm_master_open in drm_open_helper fails?

Sounds good. This can actually also happen in the failure path of 
mock_drm_getfile if anon_inode_getfile fails. I'll leave a short note 
about both of them.

Another option, and maybe cleaner, would be to move the drm_master_release
from drm_file_free into drm_close_helper. That would be fully symmetrical
and should also fix the bug here?
-Daniel

Hmmm maybe the first option to move the check out of the lock might be 
better. If I'm not wrong, we would otherwise also need to move 
drm_master_release into drm_client_close.

quoted

+
  	if (file_priv->magic)
-		idr_remove(&file_priv->master->magic_map, file_priv->magic);
+		idr_remove(&master->magic_map, file_priv->magic);
  
  	if (!drm_is_current_master_locked(file_priv))
  		goto out;

@@ -379,6 +390,7 @@ void drm_master_release(struct drm_file *file_priv)
  		drm_master_put(&file_priv->master);
  		spin_unlock(&dev->master_lookup_lock);
  	}
+unlock:
  	mutex_unlock(&dev->master_mutex);
  }
  

diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index 26f3a9ede8fe..4d029d3061d9 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c

@@ -121,6 +121,11 @@ int drm_getunique(struct drm_device *dev, void *data,
  
  	mutex_lock(&dev->master_mutex);
  	master = file_priv->master;
+	if (!master) {
+		mutex_unlock(&dev->master_mutex);
+		return -EINVAL;
+	}
+
  	if (u->unique_len >= master->unique_len) {
  		if (copy_to_user(u->unique, master->unique, master->unique_len)) {
  			mutex_unlock(&dev->master_mutex);

-- 
2.25.1