This patch eliminates the following smatch warning: drivers/gpu/drm/drm_auth.c:320 drm_master_release() warn: unlocked access 'master' (line 318) expected lock '&dev->master_mutex'
The 'file_priv->master' field should be protected by the mutex lock to '&dev->master_mutex'. This is because other processes can concurrently modify this field and free the current 'file_priv->master' pointer. This could result in a use-after-free error when 'master' is dereferenced in subsequent function calls to 'drm_legacy_lock_master_cleanup()' or to 'drm_lease_revoke()'.
An example of a scenario that would produce this error can be seen from a similar bug in 'drm_getunique()' that was reported by Syzbot: https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f80...
In the Syzbot report, another process concurrently 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.
Reported-by: Dan Carpenter dan.carpenter@oracle.com Signed-off-by: Desmond Cheong Zhi Xi desmondcheongzx@gmail.com --- drivers/gpu/drm/drm_auth.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c index f00e5abdbbf4..b59b26a71ad5 100644 --- a/drivers/gpu/drm/drm_auth.c +++ b/drivers/gpu/drm/drm_auth.c @@ -315,9 +315,10 @@ int drm_master_open(struct drm_file *file_priv) void drm_master_release(struct drm_file *file_priv) { struct drm_device *dev = file_priv->minor->dev; - struct drm_master *master = file_priv->master; + struct drm_master *master;
mutex_lock(&dev->master_mutex); + master = file_priv->master; if (file_priv->magic) idr_remove(&file_priv->master->magic_map, file_priv->magic);
On Wed, Jun 09, 2021 at 05:21:19PM +0800, Desmond Cheong Zhi Xi wrote:
This patch eliminates the following smatch warning: drivers/gpu/drm/drm_auth.c:320 drm_master_release() warn: unlocked access 'master' (line 318) expected lock '&dev->master_mutex'
The 'file_priv->master' field should be protected by the mutex lock to '&dev->master_mutex'. This is because other processes can concurrently modify this field and free the current 'file_priv->master' pointer. This could result in a use-after-free error when 'master' is dereferenced in subsequent function calls to 'drm_legacy_lock_master_cleanup()' or to 'drm_lease_revoke()'.
An example of a scenario that would produce this error can be seen from a similar bug in 'drm_getunique()' that was reported by Syzbot: https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f80...
In the Syzbot report, another process concurrently 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.
Reported-by: Dan Carpenter dan.carpenter@oracle.com Signed-off-by: Desmond Cheong Zhi Xi desmondcheongzx@gmail.com
Thanks a lot. I've done an audit of this code, and I found another potential problem in drm_is_current_master. The callers from drm_auth.c hold the dev->master_mutex, but all the external ones dont. I think we need to split this into a _locked function for use within drm_auth.c, and the exported one needs to grab the dev->master_mutex while it's checking master status. Ofc there will still be races, those are ok, but right now we run the risk of use-after free problems in drm_lease_owner.
Are you up to do that fix too?
I think the drm_lease.c code also needs an audit, there we'd need to make sure that we hold hold either the lock or a full master reference to avoid the use-after-free issues here.
Patch merged to drm-misc-fixes with cc: stable. -Daniel
drivers/gpu/drm/drm_auth.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c index f00e5abdbbf4..b59b26a71ad5 100644 --- a/drivers/gpu/drm/drm_auth.c +++ b/drivers/gpu/drm/drm_auth.c @@ -315,9 +315,10 @@ int drm_master_open(struct drm_file *file_priv) void drm_master_release(struct drm_file *file_priv) { struct drm_device *dev = file_priv->minor->dev;
- struct drm_master *master = file_priv->master;
struct drm_master *master;
mutex_lock(&dev->master_mutex);
master = file_priv->master; if (file_priv->magic) idr_remove(&file_priv->master->magic_map, file_priv->magic);
-- 2.25.1
On 10/6/21 6:10 pm, Daniel Vetter wrote:
On Wed, Jun 09, 2021 at 05:21:19PM +0800, Desmond Cheong Zhi Xi wrote:
This patch eliminates the following smatch warning: drivers/gpu/drm/drm_auth.c:320 drm_master_release() warn: unlocked access 'master' (line 318) expected lock '&dev->master_mutex'
The 'file_priv->master' field should be protected by the mutex lock to '&dev->master_mutex'. This is because other processes can concurrently modify this field and free the current 'file_priv->master' pointer. This could result in a use-after-free error when 'master' is dereferenced in subsequent function calls to 'drm_legacy_lock_master_cleanup()' or to 'drm_lease_revoke()'.
An example of a scenario that would produce this error can be seen from a similar bug in 'drm_getunique()' that was reported by Syzbot: https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f80...
In the Syzbot report, another process concurrently 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.
Reported-by: Dan Carpenter dan.carpenter@oracle.com Signed-off-by: Desmond Cheong Zhi Xi desmondcheongzx@gmail.com
Thanks a lot. I've done an audit of this code, and I found another potential problem in drm_is_current_master. The callers from drm_auth.c hold the dev->master_mutex, but all the external ones dont. I think we need to split this into a _locked function for use within drm_auth.c, and the exported one needs to grab the dev->master_mutex while it's checking master status. Ofc there will still be races, those are ok, but right now we run the risk of use-after free problems in drm_lease_owner.
Are you up to do that fix too?
Hi Daniel,
Thanks for the pointer, I'm definitely up for it!
I think the drm_lease.c code also needs an audit, there we'd need to make sure that we hold hold either the lock or a full master reference to avoid the use-after-free issues here.
I'd be happy to look into drm_lease.c as well.
Patch merged to drm-misc-fixes with cc: stable. -Daniel
drivers/gpu/drm/drm_auth.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c index f00e5abdbbf4..b59b26a71ad5 100644 --- a/drivers/gpu/drm/drm_auth.c +++ b/drivers/gpu/drm/drm_auth.c @@ -315,9 +315,10 @@ int drm_master_open(struct drm_file *file_priv) void drm_master_release(struct drm_file *file_priv) { struct drm_device *dev = file_priv->minor->dev;
- struct drm_master *master = file_priv->master;
struct drm_master *master;
mutex_lock(&dev->master_mutex);
master = file_priv->master; if (file_priv->magic) idr_remove(&file_priv->master->magic_map, file_priv->magic);
-- 2.25.1
From what I can see, there are other places in the kernel that could use the _locked version of drm_is_current_master as well, such as drm_mode_getfb in drm_framebuffer.c. I'll take a closer look, and if the changes make sense I'll prepare a patch series for them.
Best wishes, Desmond
On Thu, Jun 10, 2021 at 11:21:39PM +0800, Desmond Cheong Zhi Xi wrote:
On 10/6/21 6:10 pm, Daniel Vetter wrote:
On Wed, Jun 09, 2021 at 05:21:19PM +0800, Desmond Cheong Zhi Xi wrote:
This patch eliminates the following smatch warning: drivers/gpu/drm/drm_auth.c:320 drm_master_release() warn: unlocked access 'master' (line 318) expected lock '&dev->master_mutex'
The 'file_priv->master' field should be protected by the mutex lock to '&dev->master_mutex'. This is because other processes can concurrently modify this field and free the current 'file_priv->master' pointer. This could result in a use-after-free error when 'master' is dereferenced in subsequent function calls to 'drm_legacy_lock_master_cleanup()' or to 'drm_lease_revoke()'.
An example of a scenario that would produce this error can be seen from a similar bug in 'drm_getunique()' that was reported by Syzbot: https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f80...
In the Syzbot report, another process concurrently 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.
Reported-by: Dan Carpenter dan.carpenter@oracle.com Signed-off-by: Desmond Cheong Zhi Xi desmondcheongzx@gmail.com
Thanks a lot. I've done an audit of this code, and I found another potential problem in drm_is_current_master. The callers from drm_auth.c hold the dev->master_mutex, but all the external ones dont. I think we need to split this into a _locked function for use within drm_auth.c, and the exported one needs to grab the dev->master_mutex while it's checking master status. Ofc there will still be races, those are ok, but right now we run the risk of use-after free problems in drm_lease_owner.
Are you up to do that fix too?
Hi Daniel,
Thanks for the pointer, I'm definitely up for it!
I think the drm_lease.c code also needs an audit, there we'd need to make sure that we hold hold either the lock or a full master reference to avoid the use-after-free issues here.
I'd be happy to look into drm_lease.c as well.
Patch merged to drm-misc-fixes with cc: stable. -Daniel
drivers/gpu/drm/drm_auth.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c index f00e5abdbbf4..b59b26a71ad5 100644 --- a/drivers/gpu/drm/drm_auth.c +++ b/drivers/gpu/drm/drm_auth.c @@ -315,9 +315,10 @@ int drm_master_open(struct drm_file *file_priv) void drm_master_release(struct drm_file *file_priv) { struct drm_device *dev = file_priv->minor->dev;
- struct drm_master *master = file_priv->master;
struct drm_master *master;
mutex_lock(&dev->master_mutex);
master = file_priv->master; if (file_priv->magic) idr_remove(&file_priv->master->magic_map, file_priv->magic);
-- 2.25.1
From what I can see, there are other places in the kernel that could use the _locked version of drm_is_current_master as well, such as drm_mode_getfb in drm_framebuffer.c. I'll take a closer look, and if the changes make sense I'll prepare a patch series for them.
Oh maybe we have a naming confusion: the _locked is the one where the caller must grab the lock already, whereas drm_is_current_master would grab the master_mutex internally to do the check. The one in drm_framebuffer.c looks like it'd need the internal one since there's no other need to grab the master_mutex. -Daniel
On 11/6/21 12:48 am, Daniel Vetter wrote:
On Thu, Jun 10, 2021 at 11:21:39PM +0800, Desmond Cheong Zhi Xi wrote:
On 10/6/21 6:10 pm, Daniel Vetter wrote:
On Wed, Jun 09, 2021 at 05:21:19PM +0800, Desmond Cheong Zhi Xi wrote:
This patch eliminates the following smatch warning: drivers/gpu/drm/drm_auth.c:320 drm_master_release() warn: unlocked access 'master' (line 318) expected lock '&dev->master_mutex'
The 'file_priv->master' field should be protected by the mutex lock to '&dev->master_mutex'. This is because other processes can concurrently modify this field and free the current 'file_priv->master' pointer. This could result in a use-after-free error when 'master' is dereferenced in subsequent function calls to 'drm_legacy_lock_master_cleanup()' or to 'drm_lease_revoke()'.
An example of a scenario that would produce this error can be seen from a similar bug in 'drm_getunique()' that was reported by Syzbot: https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f80...
In the Syzbot report, another process concurrently 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.
Reported-by: Dan Carpenter dan.carpenter@oracle.com Signed-off-by: Desmond Cheong Zhi Xi desmondcheongzx@gmail.com
Thanks a lot. I've done an audit of this code, and I found another potential problem in drm_is_current_master. The callers from drm_auth.c hold the dev->master_mutex, but all the external ones dont. I think we need to split this into a _locked function for use within drm_auth.c, and the exported one needs to grab the dev->master_mutex while it's checking master status. Ofc there will still be races, those are ok, but right now we run the risk of use-after free problems in drm_lease_owner.
Are you up to do that fix too?
Hi Daniel,
Thanks for the pointer, I'm definitely up for it!
I think the drm_lease.c code also needs an audit, there we'd need to make sure that we hold hold either the lock or a full master reference to avoid the use-after-free issues here.
I'd be happy to look into drm_lease.c as well.
Patch merged to drm-misc-fixes with cc: stable. -Daniel
drivers/gpu/drm/drm_auth.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c index f00e5abdbbf4..b59b26a71ad5 100644 --- a/drivers/gpu/drm/drm_auth.c +++ b/drivers/gpu/drm/drm_auth.c @@ -315,9 +315,10 @@ int drm_master_open(struct drm_file *file_priv) void drm_master_release(struct drm_file *file_priv) { struct drm_device *dev = file_priv->minor->dev;
- struct drm_master *master = file_priv->master;
struct drm_master *master;
mutex_lock(&dev->master_mutex);
master = file_priv->master; if (file_priv->magic) idr_remove(&file_priv->master->magic_map, file_priv->magic);
-- 2.25.1
From what I can see, there are other places in the kernel that could use the _locked version of drm_is_current_master as well, such as drm_mode_getfb in drm_framebuffer.c. I'll take a closer look, and if the changes make sense I'll prepare a patch series for them.
Oh maybe we have a naming confusion: the _locked is the one where the caller must grab the lock already, whereas drm_is_current_master would grab the master_mutex internally to do the check. The one in drm_framebuffer.c looks like it'd need the internal one since there's no other need to grab the master_mutex. -Daniel
Ah ok got it, I think I confused myself earlier.
Just to check, may I include you in a Reported-by: tag?
On Fri, Jun 11, 2021 at 4:18 AM Desmond Cheong Zhi Xi desmondcheongzx@gmail.com wrote: On 11/6/21 12:48 am, Daniel Vetter wrote:
On Thu, Jun 10, 2021 at 11:21:39PM +0800, Desmond Cheong Zhi Xi wrote:
On 10/6/21 6:10 pm, Daniel Vetter wrote:
On Wed, Jun 09, 2021 at 05:21:19PM +0800, Desmond Cheong Zhi Xi wrote:
This patch eliminates the following smatch warning: drivers/gpu/drm/drm_auth.c:320 drm_master_release() warn: unlocked access 'master' (line 318) expected lock '&dev->master_mutex'
The 'file_priv->master' field should be protected by the mutex lock to '&dev->master_mutex'. This is because other processes can concurrently modify this field and free the current 'file_priv->master' pointer. This could result in a use-after-free error when 'master' is dereferenced in subsequent function calls to 'drm_legacy_lock_master_cleanup()' or to 'drm_lease_revoke()'.
An example of a scenario that would produce this error can be seen from a similar bug in 'drm_getunique()' that was reported by Syzbot: https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f80...
In the Syzbot report, another process concurrently 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.
Reported-by: Dan Carpenter dan.carpenter@oracle.com Signed-off-by: Desmond Cheong Zhi Xi desmondcheongzx@gmail.com
Thanks a lot. I've done an audit of this code, and I found another potential problem in drm_is_current_master. The callers from drm_auth.c hold the dev->master_mutex, but all the external ones dont. I think we need to split this into a _locked function for use within drm_auth.c, and the exported one needs to grab the dev->master_mutex while it's checking master status. Ofc there will still be races, those are ok, but right now we run the risk of use-after free problems in drm_lease_owner.
Are you up to do that fix too?
Hi Daniel,
Thanks for the pointer, I'm definitely up for it!
I think the drm_lease.c code also needs an audit, there we'd need to make sure that we hold hold either the lock or a full master reference to avoid the use-after-free issues here.
I'd be happy to look into drm_lease.c as well.
Patch merged to drm-misc-fixes with cc: stable. -Daniel
drivers/gpu/drm/drm_auth.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c index f00e5abdbbf4..b59b26a71ad5 100644 --- a/drivers/gpu/drm/drm_auth.c +++ b/drivers/gpu/drm/drm_auth.c @@ -315,9 +315,10 @@ int drm_master_open(struct drm_file *file_priv) void drm_master_release(struct drm_file *file_priv) { struct drm_device *dev = file_priv->minor->dev;
- struct drm_master *master = file_priv->master;
struct drm_master *master;
mutex_lock(&dev->master_mutex);master = file_priv->master; if (file_priv->magic) idr_remove(&file_priv->master->magic_map, file_priv->magic);
-- 2.25.1
From what I can see, there are other places in the kernel that could use the _locked version of drm_is_current_master as well, such as drm_mode_getfb in drm_framebuffer.c. I'll take a closer look, and if the changes make sense I'll prepare a patch series for them.
Oh maybe we have a naming confusion: the _locked is the one where the caller must grab the lock already, whereas drm_is_current_master would grab the master_mutex internally to do the check. The one in drm_framebuffer.c looks like it'd need the internal one since there's no other need to grab the master_mutex. -Daniel
Ah ok got it, I think I confused myself earlier.
Just to check, may I include you in a Reported-by: tag?
Sure. -Daniel
On Thu, 10 Jun 2021 at 11:10, Daniel Vetter daniel@ffwll.ch wrote:
On Wed, Jun 09, 2021 at 05:21:19PM +0800, Desmond Cheong Zhi Xi wrote:
This patch eliminates the following smatch warning: drivers/gpu/drm/drm_auth.c:320 drm_master_release() warn: unlocked access 'master' (line 318) expected lock '&dev->master_mutex'
The 'file_priv->master' field should be protected by the mutex lock to '&dev->master_mutex'. This is because other processes can concurrently modify this field and free the current 'file_priv->master' pointer. This could result in a use-after-free error when 'master' is dereferenced in subsequent function calls to 'drm_legacy_lock_master_cleanup()' or to 'drm_lease_revoke()'.
An example of a scenario that would produce this error can be seen from a similar bug in 'drm_getunique()' that was reported by Syzbot: https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f80...
In the Syzbot report, another process concurrently 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.
Reported-by: Dan Carpenter dan.carpenter@oracle.com Signed-off-by: Desmond Cheong Zhi Xi desmondcheongzx@gmail.com
Thanks a lot. I've done an audit of this code, and I found another potential problem in drm_is_current_master. The callers from drm_auth.c hold the dev->master_mutex, but all the external ones dont. I think we need to split this into a _locked function for use within drm_auth.c, and the exported one needs to grab the dev->master_mutex while it's checking master status. Ofc there will still be races, those are ok, but right now we run the risk of use-after free problems in drm_lease_owner.
Note that some code does acquire the mutex via drm_master_internal_acquire - so we should be careful. As mentioned elsewhere - having a _locked version of drm_is_current_master sounds good.
Might as well throw a lockdep_assert_held_once in there just in case :-P
Happy to help review the follow-up patches. -Emil
On 11/6/21 1:49 am, Emil Velikov wrote:
On Thu, 10 Jun 2021 at 11:10, Daniel Vetter daniel@ffwll.ch wrote:
On Wed, Jun 09, 2021 at 05:21:19PM +0800, Desmond Cheong Zhi Xi wrote:
This patch eliminates the following smatch warning: drivers/gpu/drm/drm_auth.c:320 drm_master_release() warn: unlocked access 'master' (line 318) expected lock '&dev->master_mutex'
The 'file_priv->master' field should be protected by the mutex lock to '&dev->master_mutex'. This is because other processes can concurrently modify this field and free the current 'file_priv->master' pointer. This could result in a use-after-free error when 'master' is dereferenced in subsequent function calls to 'drm_legacy_lock_master_cleanup()' or to 'drm_lease_revoke()'.
An example of a scenario that would produce this error can be seen from a similar bug in 'drm_getunique()' that was reported by Syzbot: https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f80...
In the Syzbot report, another process concurrently 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.
Reported-by: Dan Carpenter dan.carpenter@oracle.com Signed-off-by: Desmond Cheong Zhi Xi desmondcheongzx@gmail.com
Thanks a lot. I've done an audit of this code, and I found another potential problem in drm_is_current_master. The callers from drm_auth.c hold the dev->master_mutex, but all the external ones dont. I think we need to split this into a _locked function for use within drm_auth.c, and the exported one needs to grab the dev->master_mutex while it's checking master status. Ofc there will still be races, those are ok, but right now we run the risk of use-after free problems in drm_lease_owner.
Note that some code does acquire the mutex via drm_master_internal_acquire - so we should be careful. As mentioned elsewhere - having a _locked version of drm_is_current_master sounds good.
Might as well throw a lockdep_assert_held_once in there just in case :-P
Happy to help review the follow-up patches. -Emil
Thanks for the advice, Emil!
I did a preliminary check on the code that calls drm_master_internal_acquire in drm_client_modeset.c and drm_fb_helper.c, and it doesn't seem like they eventually call drm_is_current_master. So we should be good on that front.
lockdep_assert_held_once sounds good :)
Best wishes, Desmond
dri-devel@lists.freedesktop.org
-
Daniel Vetter -
Desmond Cheong Zhi Xi -
Emil Velikov