[PATCH 0/2] drm: Address potential UAF bugs with drm_master ptrs

[PATCH 0/2] drm: Address potential UAF bugs with drm_master ptrs

[Desmond Cheong Zhi Xi]

This patch series addresses potential use-after-free errors when dereferencing pointers to struct drm_master. These were identified after one such bug was caught by Syzbot in drm_getunique():
https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f803

The series is broken up into two patches:

1. Implement a locked version of drm_is_current_master() function that's used within drm_auth.c

2. Identify areas in drm_lease.c where pointers to struct drm_master are dereferenced, and ensure that the master pointers are protected by a mutex

Desmond Cheong Zhi Xi (2):
  drm: Add a locked version of drm_is_current_master
  drm: Protect drm_master pointers in drm_lease.c

 drivers/gpu/drm/drm_auth.c  | 23 ++++++++++++---
 drivers/gpu/drm/drm_lease.c | 58 +++++++++++++++++++++++++++----------
 2 files changed, 62 insertions(+), 19 deletions(-)

-- 
2.25.1

[PATCH 1/2] drm: Add a locked version of drm_is_current_master

[Desmond Cheong Zhi Xi]

While checking the master status of the DRM file in
drm_is_current_master(), the device's master mutex should be
held. Without the mutex, the pointer fpriv->master may be freed
concurrently by another process calling drm_setmaster_ioctl(). This
could lead to use-after-free errors when the pointer is subsequently
dereferenced in drm_lease_owner().

The callers of drm_is_current_master() from drm_auth.c hold the
device's master mutex, but external callers do not. Hence, we implement
drm_is_current_master_locked() to be used within drm_auth.c, and
modify drm_is_current_master() to grab the device's master mutex
before checking the master status.

Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
---
 drivers/gpu/drm/drm_auth.c | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index 232abbba3686..c6bf52c310a9 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -61,6 +61,8 @@
  * trusted clients.
  */
 
+static bool drm_is_current_master_locked(struct drm_file *fpriv);
+
 int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv)
 {
 	struct drm_auth *auth = data;
@@ -223,7 +225,7 @@ int drm_setmaster_ioctl(struct drm_device *dev, void *data,
 	if (ret)
 		goto out_unlock;
 
-	if (drm_is_current_master(file_priv))
+	if (drm_is_current_master_locked(file_priv))
 		goto out_unlock;
 
 	if (dev->master) {
@@ -272,7 +274,7 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
 	if (ret)
 		goto out_unlock;
 
-	if (!drm_is_current_master(file_priv)) {
+	if (!drm_is_current_master_locked(file_priv)) {
 		ret = -EINVAL;
 		goto out_unlock;
 	}
@@ -321,7 +323,7 @@ void drm_master_release(struct drm_file *file_priv)
 	if (file_priv->magic)
 		idr_remove(&file_priv->master->magic_map, file_priv->magic);
 
-	if (!drm_is_current_master(file_priv))
+	if (!drm_is_current_master_locked(file_priv))
 		goto out;
 
 	drm_legacy_lock_master_cleanup(dev, master);
@@ -342,6 +344,13 @@ void drm_master_release(struct drm_file *file_priv)
 	mutex_unlock(&dev->master_mutex);
 }
 
+static bool drm_is_current_master_locked(struct drm_file *fpriv)
+{
+	lockdep_assert_held_once(&fpriv->master->dev->master_mutex);
+
+	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
+}
+
 /**
  * drm_is_current_master - checks whether @priv is the current master
  * @fpriv: DRM file private
@@ -354,7 +363,13 @@ void drm_master_release(struct drm_file *file_priv)
  */
 bool drm_is_current_master(struct drm_file *fpriv)
 {
-	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
+	bool ret;
+
+	mutex_lock(&fpriv->master->dev->master_mutex);
+	ret = drm_is_current_master_locked(fpriv);
+	mutex_unlock(&fpriv->master->dev->master_mutex);
+
+	return ret;
 }
 EXPORT_SYMBOL(drm_is_current_master);
 
-- 
2.25.1

[PATCH 2/2] drm: Protect drm_master pointers in drm_lease.c

[Desmond Cheong Zhi Xi]

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@ffwll.ch>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
---
 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..f2422f1c4915 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);
+	ret = _drm_lease_held_master(file_priv->master, id);
+	mutex_unlock(&file_priv->master->dev->master_mutex);
+
+	return ret;
 }
 
 /**
@@ -132,10 +138,12 @@ bool drm_lease_held(struct drm_file *file_priv, int id)
 	if (!file_priv || !file_priv->master || !file_priv->master->lessor)
 		return true;
 
+	mutex_lock(&file_priv->master->dev->master_mutex);
 	master = file_priv->master;
 	mutex_lock(&master->dev->mode_config.idr_mutex);
 	ret = _drm_lease_held_master(master, id);
 	mutex_unlock(&master->dev->mode_config.idr_mutex);
+	mutex_unlock(&file_priv->master->dev->master_mutex);
 	return ret;
 }
 
@@ -158,6 +166,7 @@ uint32_t drm_lease_filter_crtcs(struct drm_file *file_priv, uint32_t crtcs_in)
 	if (!file_priv || !file_priv->master || !file_priv->master->lessor)
 		return crtcs_in;
 
+	mutex_lock(&file_priv->master->dev->master_mutex);
 	master = file_priv->master;
 	dev = master->dev;
 
@@ -177,6 +186,7 @@ uint32_t drm_lease_filter_crtcs(struct drm_file *file_priv, uint32_t crtcs_in)
 		count_in++;
 	}
 	mutex_unlock(&master->dev->mode_config.idr_mutex);
+	mutex_unlock(&file_priv->master->dev->master_mutex);
 	return crtcs_out;
 }
 
@@ -490,7 +500,7 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
 	size_t object_count;
 	int ret = 0;
 	struct idr leases;
-	struct drm_master *lessor = lessor_priv->master;
+	struct drm_master *lessor;
 	struct drm_master *lessee = NULL;
 	struct file *lessee_file = NULL;
 	struct file *lessor_file = lessor_priv->filp;
@@ -502,12 +512,6 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
 	if (!drm_core_check_feature(dev, DRIVER_MODESET))
 		return -EOPNOTSUPP;
 
-	/* Do not allow sub-leases */
-	if (lessor->lessor) {
-		DRM_DEBUG_LEASE("recursive leasing not allowed\n");
-		return -EINVAL;
-	}
-
 	/* need some objects */
 	if (cl->object_count == 0) {
 		DRM_DEBUG_LEASE("no objects in lease\n");
@@ -519,12 +523,23 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
 		return -EINVAL;
 	}
 
+	mutex_lock(&dev->master_mutex);
+	lessor = lessor_priv->master;
+	/* Do not allow sub-leases */
+	if (lessor->lessor) {
+		DRM_DEBUG_LEASE("recursive leasing not allowed\n");
+		ret = -EINVAL;
+		goto unlock;
+	}
+
 	object_count = cl->object_count;
 
 	object_ids = memdup_user(u64_to_user_ptr(cl->object_ids),
 			array_size(object_count, sizeof(__u32)));
-	if (IS_ERR(object_ids))
-		return PTR_ERR(object_ids);
+	if (IS_ERR(object_ids)) {
+		ret = PTR_ERR(object_ids);
+		goto unlock;
+	}
 
 	idr_init(&leases);
 
@@ -535,14 +550,15 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
 	if (ret) {
 		DRM_DEBUG_LEASE("lease object lookup failed: %i\n", ret);
 		idr_destroy(&leases);
-		return ret;
+		goto unlock;
 	}
 
 	/* Allocate a file descriptor for the lease */
 	fd = get_unused_fd_flags(cl->flags & (O_CLOEXEC | O_NONBLOCK));
 	if (fd < 0) {
 		idr_destroy(&leases);
-		return fd;
+		ret = fd;
+		goto unlock;
 	}
 
 	DRM_DEBUG_LEASE("Creating lease\n");
@@ -578,6 +594,7 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
 	/* Hook up the fd */
 	fd_install(fd, lessee_file);
 
+	mutex_unlock(&dev->master_mutex);
 	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl succeeded\n");
 	return 0;
 
@@ -587,6 +604,8 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
 out_leases:
 	put_unused_fd(fd);
 
+unlock:
+	mutex_unlock(&dev->master_mutex);
 	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl failed: %d\n", ret);
 	return ret;
 }
@@ -609,7 +628,7 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
 	struct drm_mode_list_lessees *arg = data;
 	__u32 __user *lessee_ids = (__u32 __user *) (uintptr_t) (arg->lessees_ptr);
 	__u32 count_lessees = arg->count_lessees;
-	struct drm_master *lessor = lessor_priv->master, *lessee;
+	struct drm_master *lessor, *lessee;
 	int count;
 	int ret = 0;
 
@@ -620,6 +639,8 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
 	if (!drm_core_check_feature(dev, DRIVER_MODESET))
 		return -EOPNOTSUPP;
 
+	mutex_lock(&dev->master_mutex);
+	lessor = lessor_priv->master;
 	DRM_DEBUG_LEASE("List lessees for %d\n", lessor->lessee_id);
 
 	mutex_lock(&dev->mode_config.idr_mutex);
@@ -643,6 +664,7 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
 		arg->count_lessees = count;
 
 	mutex_unlock(&dev->mode_config.idr_mutex);
+	mutex_unlock(&dev->master_mutex);
 
 	return ret;
 }
@@ -662,7 +684,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
 	struct drm_mode_get_lease *arg = data;
 	__u32 __user *object_ids = (__u32 __user *) (uintptr_t) (arg->objects_ptr);
 	__u32 count_objects = arg->count_objects;
-	struct drm_master *lessee = lessee_priv->master;
+	struct drm_master *lessee;
 	struct idr *object_idr;
 	int count;
 	void *entry;
@@ -678,8 +700,10 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
 
 	DRM_DEBUG_LEASE("get lease for %d\n", lessee->lessee_id);
 
+	mutex_lock(&dev->master_mutex);
 	mutex_lock(&dev->mode_config.idr_mutex);
 
+	lessee = lessee_priv->master;
 	if (lessee->lessor == NULL)
 		/* owner can use all objects */
 		object_idr = &lessee->dev->mode_config.object_idr;
@@ -703,6 +727,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
 		arg->count_objects = count;
 
 	mutex_unlock(&dev->mode_config.idr_mutex);
+	mutex_unlock(&dev->master_mutex);
 
 	return ret;
 }
@@ -721,7 +746,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
 				void *data, struct drm_file *lessor_priv)
 {
 	struct drm_mode_revoke_lease *arg = data;
-	struct drm_master *lessor = lessor_priv->master;
+	struct drm_master *lessor;
 	struct drm_master *lessee;
 	int ret = 0;
 
@@ -731,8 +756,10 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
 	if (!drm_core_check_feature(dev, DRIVER_MODESET))
 		return -EOPNOTSUPP;
 
+	mutex_lock(&dev->master_mutex);
 	mutex_lock(&dev->mode_config.idr_mutex);
 
+	lessor = lessor_priv->master;
 	lessee = _drm_find_lessee(lessor, arg->lessee_id);
 
 	/* No such lessee */
@@ -751,6 +778,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
 
 fail:
 	mutex_unlock(&dev->mode_config.idr_mutex);
+	mutex_unlock(&dev->master_mutex);
 
 	return ret;
 }
-- 
2.25.1

Re: [PATCH 1/2] drm: Add a locked version of drm_is_current_master

[Emil Velikov]

On Sat, 12 Jun 2021 at 13:55, Desmond Cheong Zhi Xi
<desmondcheongzx@gmail.com> wrote:
>
> While checking the master status of the DRM file in
> drm_is_current_master(), the device's master mutex should be
> held. Without the mutex, the pointer fpriv->master may be freed
> concurrently by another process calling drm_setmaster_ioctl(). This
> could lead to use-after-free errors when the pointer is subsequently
> dereferenced in drm_lease_owner().
>
> The callers of drm_is_current_master() from drm_auth.c hold the
> device's master mutex, but external callers do not. Hence, we implement
> drm_is_current_master_locked() to be used within drm_auth.c, and
> modify drm_is_current_master() to grab the device's master mutex
> before checking the master status.
>
> Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>

-Emil

Re: [PATCH 2/2] drm: Protect drm_master pointers in drm_lease.c

[Emil Velikov]

On Sat, 12 Jun 2021 at 13:55, Desmond Cheong Zhi Xi
<desmondcheongzx@gmail.com> 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@ffwll.ch>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
> ---

<snip>

> @@ -578,6 +594,7 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>         /* Hook up the fd */
>         fd_install(fd, lessee_file);
>
> +       mutex_unlock(&dev->master_mutex);

I was going to suggest pushing the unlock call further up - after the
drm_lease_create call. Although that would make the error path rather
messy, so let's keep it as-is.

<snip>

> @@ -662,7 +684,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
>         struct drm_mode_get_lease *arg = data;
>         __u32 __user *object_ids = (__u32 __user *) (uintptr_t) (arg->objects_ptr);
>         __u32 count_objects = arg->count_objects;
> -       struct drm_master *lessee = lessee_priv->master;
> +       struct drm_master *lessee;
>         struct idr *object_idr;
>         int count;
>         void *entry;
> @@ -678,8 +700,10 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
>
>         DRM_DEBUG_LEASE("get lease for %d\n", lessee->lessee_id);
>
> +       mutex_lock(&dev->master_mutex);

As-is we're dereferencing an uninitialised pointer in DRM_DEBUG_LEASE.
Move the lock and assignment before the DRM_DEBUG_LEASE, just like
you've done in the list ioctl.

With this fix, the patch is;
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>

-Emil

Re: [PATCH 2/2] drm: Protect drm_master pointers in drm_lease.c

[Desmond Cheong Zhi Xi]

On 15/6/21 3:41 am, Emil Velikov wrote:
> On Sat, 12 Jun 2021 at 13:55, Desmond Cheong Zhi Xi
> <desmondcheongzx@gmail.com> 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@ffwll.ch>
>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
>> ---
> 
> <snip>
> 
>> @@ -578,6 +594,7 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>>          /* Hook up the fd */
>>          fd_install(fd, lessee_file);
>>
>> +       mutex_unlock(&dev->master_mutex);
> 
> I was going to suggest pushing the unlock call further up - after the
> drm_lease_create call. Although that would make the error path rather
> messy, so let's keep it as-is.
> 
> <snip>
> 
>> @@ -662,7 +684,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
>>          struct drm_mode_get_lease *arg = data;
>>          __u32 __user *object_ids = (__u32 __user *) (uintptr_t) (arg->objects_ptr);
>>          __u32 count_objects = arg->count_objects;
>> -       struct drm_master *lessee = lessee_priv->master;
>> +       struct drm_master *lessee;
>>          struct idr *object_idr;
>>          int count;
>>          void *entry;
>> @@ -678,8 +700,10 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
>>
>>          DRM_DEBUG_LEASE("get lease for %d\n", lessee->lessee_id);
>>
>> +       mutex_lock(&dev->master_mutex);
> 
> As-is we're dereferencing an uninitialised pointer in DRM_DEBUG_LEASE.
> Move the lock and assignment before the DRM_DEBUG_LEASE, just like
> you've done in the list ioctl.
> 
> With this fix, the patch is;
> Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
> 
> -Emil
> 


Good catch, thanks for the feedback Emil. I'll fix this up in a v2 patchset.

Best wishes,
Desmond