Messages in this thread | | | Subject | Re: [PATCH] drm: Lock pointer access in drm_master_release() | From | Desmond Cheong Zhi Xi <> | Date | Thu, 10 Jun 2021 23:21:39 +0800 |
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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
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