linux-kernel - Re: Use of copy_from_user in msm_gem_submit.c while holding a spin

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Date:	Thu, 18 Aug 2016 15:08:45 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Rob Clark <robdclark@...il.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Vaishali Thakkar <vaishali.thakkar@...cle.com>,
	David Airlie <airlied@...ux.ie>,
	linux-arm-msm <linux-arm-msm@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	freedreno@...ts.freedesktop.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Julia Lawall <julia.lawall@...6.fr>,
	Daniel Vetter <daniel@...ll.ch>
Subject: Re: Use of copy_from_user in msm_gem_submit.c while holding a
 spin_lock

On Thu, Aug 18, 2016 at 06:55:12AM -0400, Rob Clark wrote:
> On Thu, Aug 18, 2016 at 4:36 AM, Daniel Vetter <daniel@...ll.ch> wrote:
> > On Wed, Aug 17, 2016 at 05:29:31PM -0400, Rob Clark wrote:
> >> diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
> >> index 6cd4af4..4502e4b 100644
> >> --- a/drivers/gpu/drm/msm/msm_gem.c
> >> +++ b/drivers/gpu/drm/msm/msm_gem.c
> >> @@ -201,6 +201,13 @@ int msm_gem_fault(struct vm_area_struct *vma,
> >> struct vm_fault *vmf)
> >>      pgoff_t pgoff;
> >>      int ret;
> >>
> >> +    /* I think this should only happen if userspace tries to pass a
> >> +     * mmap'd but unfaulted gem bo vaddr into submit ioctl, triggering
> >> +     * a page fault while struct_mutex is already held
> >> +     */
> >> +    if (mutex_is_locked_by(&dev->struct_mutex, current))
> >> +        return VM_FAULT_SIGBUS;
> >
> > This is an ok (well still horrible) heuristics for the shrinker, but for
> > correctness it kinda doesn't cut it. What you need to do instead is drop
> > all the locks, copy relocations into a temp memory area and then proceed
> > in the msm command submission path above.
> >
> > Also reentrant mutexes are evil ;-)
> 
> Please note that this is not a reentrant mutex in the fault path, it
> bails with VM_FAULT_SIGBUG!

Except on UP it totally deadlocks ;-)
-Daniel

> There is never a legit reason to use a gem bo for the bos (or cmds)
> table in the ioctl, so while this may not be pretty, I believe it is
> an acceptable solution.
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch