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
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 <[email protected]>
Signed-off-by: Desmond Cheong Zhi Xi <[email protected]>
---
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
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 <[email protected]>
Signed-off-by: Desmond Cheong Zhi Xi <[email protected]>
---
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
On Sat, 12 Jun 2021 at 13:55, Desmond Cheong Zhi Xi
<[email protected]> 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 <[email protected]>
> Signed-off-by: Desmond Cheong Zhi Xi <[email protected]>
Reviewed-by: Emil Velikov <[email protected]>
-Emil
On Sat, 12 Jun 2021 at 13:55, Desmond Cheong Zhi Xi
<[email protected]> 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 <[email protected]>
> Signed-off-by: Desmond Cheong Zhi Xi <[email protected]>
> ---
<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 <[email protected]>
-Emil
On 15/6/21 3:41 am, Emil Velikov wrote:
> On Sat, 12 Jun 2021 at 13:55, Desmond Cheong Zhi Xi
> <[email protected]> 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 <[email protected]>
>> Signed-off-by: Desmond Cheong Zhi Xi <[email protected]>
>> ---
>
> <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 <[email protected]>
>
> -Emil
>
Good catch, thanks for the feedback Emil. I'll fix this up in a v2 patchset.
Best wishes,
Desmond