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 09:14:30 -0400
From:	Rob Clark <robdclark@...il.com>
To:	Rob Clark <robdclark@...il.com>, 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>
Cc:	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 9:08 AM, Daniel Vetter <daniel@...ll.ch> wrote:
> 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 ;-)

except UP does not exist ;-)

like I mentioned, I can add 'depends SMP' to kconfig

BR,
-R

> -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