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Thu, 10 Jun 2021 19:18:27 -0700 (PDT) Subject: Re: [PATCH] drm: Lock pointer access in drm_master_release() To: maarten.lankhorst@linux.intel.com, mripard@kernel.org, tzimmermann@suse.de, airlied@linux.ie, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, skhan@linuxfoundation.org, gregkh@linuxfoundation.org, linux-kernel-mentees@lists.linuxfoundation.org, Dan Carpenter References: <20210609092119.173590-1-desmondcheongzx@gmail.com> From: Desmond Cheong Zhi Xi Message-ID: Date: Fri, 11 Jun 2021 10:18:22 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=windows-1252; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" 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 >>>> Signed-off-by: Desmond Cheong Zhi Xi >>> >>> 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?