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

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

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.

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 desmondcheongzx@gmail.com

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?

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

• 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