There is a time-of-check-to-time-of-use error in drm_getunique() due
to retrieving file_priv->master prior to locking the device's master
mutex.
An example can be seen in the crash report of the use-after-free error
found by Syzbot:
https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f803
In the report, the master pointer was used after being freed. This is
because another process had acquired the device's master mutex in
drm_setmaster_ioctl(), then overwrote fpriv->master in
drm_new_set_master(). The old value of fpriv->master was subsequently
freed before the mutex was unlocked.
To fix this, we lock the device's master mutex before retrieving the
pointer from from fpriv->master. This patch passes the Syzbot
reproducer test.
Reported-by: [email protected]
Signed-off-by: Desmond Cheong Zhi Xi <[email protected]>
---
In this patch, I made the assumption that dev and file_priv->master->dev refer to the same struct drm_device*, and from my test runs this seemed to be the case. If this was a wrong assumption to make, please let me know.
drivers/gpu/drm/drm_ioctl.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)
diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index d273d1a8603a..495a4767a443 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -118,17 +118,18 @@ int drm_getunique(struct drm_device *dev, void *data,
struct drm_file *file_priv)
{
struct drm_unique *u = data;
- struct drm_master *master = file_priv->master;
+ struct drm_master *master;
- mutex_lock(&master->dev->master_mutex);
+ mutex_lock(&dev->master_mutex);
+ master = file_priv->master;
if (u->unique_len >= master->unique_len) {
if (copy_to_user(u->unique, master->unique, master->unique_len)) {
- mutex_unlock(&master->dev->master_mutex);
+ mutex_unlock(&dev->master_mutex);
return -EFAULT;
}
}
u->unique_len = master->unique_len;
- mutex_unlock(&master->dev->master_mutex);
+ mutex_unlock(&dev->master_mutex);
return 0;
}
--
2.25.1
On Tue, Jun 08, 2021 at 07:04:36PM +0800, Desmond Cheong Zhi Xi wrote:
> There is a time-of-check-to-time-of-use error in drm_getunique() due
> to retrieving file_priv->master prior to locking the device's master
> mutex.
>
> An example can be seen in the crash report of the use-after-free error
> found by Syzbot:
> https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f803
>
> In the report, the master pointer was used after being freed. This is
> because another process had acquired the device's master mutex in
> drm_setmaster_ioctl(), then overwrote fpriv->master in
> drm_new_set_master(). The old value of fpriv->master was subsequently
> freed before the mutex was unlocked.
>
> To fix this, we lock the device's master mutex before retrieving the
> pointer from from fpriv->master. This patch passes the Syzbot
> reproducer test.
>
> Reported-by: [email protected]
> Signed-off-by: Desmond Cheong Zhi Xi <[email protected]>
Very nice debug work, thanks a lot!
> ---
>
> In this patch, I made the assumption that dev and file_priv->master->dev
> refer to the same struct drm_device*, and from my test runs this seemed
> to be the case. If this was a wrong assumption to make, please let me
> know.
Yup that's correct.
Patch applied to drm-misc-fixes and cc: stable added so it gets
backported.
Cheers, Daniel
>
> drivers/gpu/drm/drm_ioctl.c | 9 +++++----
> 1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
> index d273d1a8603a..495a4767a443 100644
> --- a/drivers/gpu/drm/drm_ioctl.c
> +++ b/drivers/gpu/drm/drm_ioctl.c
> @@ -118,17 +118,18 @@ int drm_getunique(struct drm_device *dev, void *data,
> struct drm_file *file_priv)
> {
> struct drm_unique *u = data;
> - struct drm_master *master = file_priv->master;
> + struct drm_master *master;
>
> - mutex_lock(&master->dev->master_mutex);
> + mutex_lock(&dev->master_mutex);
> + master = file_priv->master;
> if (u->unique_len >= master->unique_len) {
> if (copy_to_user(u->unique, master->unique, master->unique_len)) {
> - mutex_unlock(&master->dev->master_mutex);
> + mutex_unlock(&dev->master_mutex);
> return -EFAULT;
> }
> }
> u->unique_len = master->unique_len;
> - mutex_unlock(&master->dev->master_mutex);
> + mutex_unlock(&dev->master_mutex);
>
> return 0;
> }
> --
> 2.25.1
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch