[PATCH] drm: Lock pointer access in drm_master_release()

[PATCH] drm: Lock pointer access in drm_master_release()

[Desmond Cheong Zhi Xi]

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=148d2f1dfac64af52ffd27b661981a540724f803

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);
 
-- 
2.25.1

Re: [PATCH] drm: Lock pointer access in drm_master_release()

[Daniel Vetter]

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=148d2f1dfac64af52ffd27b661981a540724f803
> 
> 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
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Re: [PATCH] drm: Lock pointer access in drm_master_release()

[Desmond Cheong Zhi Xi]

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=148d2f1dfac64af52ffd27b661981a540724f803
>>
>> 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

Re: [PATCH] drm: Lock pointer access in drm_master_release()

[Daniel Vetter]

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=148d2f1dfac64af52ffd27b661981a540724f803
> > > 
> > > 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
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Re: [PATCH] drm: Lock pointer access in drm_master_release()

[Desmond Cheong Zhi Xi]

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=148d2f1dfac64af52ffd27b661981a540724f803
>>>>
>>>> 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?

Re: [PATCH] drm: Lock pointer access in drm_master_release()

[Daniel Vetter]

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=148d2f1dfac64af52ffd27b661981a540724f803
> >>>>
> >>>> 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
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Re: [PATCH] drm: Lock pointer access in drm_master_release()

[Emil Velikov]

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=148d2f1dfac64af52ffd27b661981a540724f803
> >
> > 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

Re: [PATCH] drm: Lock pointer access in drm_master_release()

[Desmond Cheong Zhi Xi]

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=148d2f1dfac64af52ffd27b661981a540724f803
>>>
>>> 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