[Intel-gfx] [PATCH v2 3/3] drm/i915/userptr: Disallow wrapping GTT into a userptr

Wed Mar 8 13:04:29 UTC 2017

On 08/03/2017 10:01, Chris Wilson wrote:
> On Wed, Mar 08, 2017 at 08:25:12AM +0000, Tvrtko Ursulin wrote:
>>
>> On 07/03/2017 20:58, Chris Wilson wrote:
>>> If we allow the user to convert a GTT mmap address into a userptr, we
>>> may end up in recursion hell, where currently we hit a mutex deadlock
>>> but other possibilities include use-after-free during the
>>> unbind/cancel_userptr.
>>>
>>> [  143.203989] gem_userptr_bli D    0   902    898 0x00000000
>>> [  143.204054] Call Trace:
>>> [  143.204137]  __schedule+0x511/0x1180
>>> [  143.204195]  ? pci_mmcfg_check_reserved+0xc0/0xc0
>>> [  143.204274]  schedule+0x57/0xe0
>>> [  143.204327]  schedule_timeout+0x383/0x670
>>> [  143.204374]  ? trace_hardirqs_on_caller+0x187/0x280
>>> [  143.204457]  ? trace_hardirqs_on_thunk+0x1a/0x1c
>>> [  143.204507]  ? usleep_range+0x110/0x110
>>> [  143.204657]  ? irq_exit+0x89/0x100
>>> [  143.204710]  ? retint_kernel+0x2d/0x2d
>>> [  143.204794]  ? trace_hardirqs_on_caller+0x187/0x280
>>> [  143.204857]  ? _raw_spin_unlock_irq+0x33/0x60
>>> [  143.204944]  wait_for_common+0x1f0/0x2f0
>>> [  143.205006]  ? out_of_line_wait_on_atomic_t+0x170/0x170
>>> [  143.205103]  ? wake_up_q+0xa0/0xa0
>>> [  143.205159]  ? flush_workqueue_prep_pwqs+0x15a/0x2c0
>>> [  143.205237]  wait_for_completion+0x1d/0x20
>>> [  143.205292]  flush_workqueue+0x2e9/0xbb0
>>> [  143.205339]  ? flush_workqueue+0x163/0xbb0
>>> [  143.205418]  ? __schedule+0x533/0x1180
>>> [  143.205498]  ? check_flush_dependency+0x1a0/0x1a0
>>> [  143.205681]  i915_gem_userptr_mn_invalidate_range_start+0x1c7/0x270 [i915]
>>> [  143.205865]  ? i915_gem_userptr_dmabuf_export+0x40/0x40 [i915]
>>> [  143.205955]  __mmu_notifier_invalidate_range_start+0xc6/0x120
>>> [  143.206044]  ? __mmu_notifier_invalidate_range_start+0x51/0x120
>>> [  143.206123]  zap_page_range_single+0x1c7/0x1f0
>>> [  143.206171]  ? unmap_single_vma+0x160/0x160
>>> [  143.206260]  ? unmap_mapping_range+0xa9/0x1b0
>>> [  143.206308]  ? vma_interval_tree_subtree_search+0x75/0xd0
>>> [  143.206397]  unmap_mapping_range+0x18f/0x1b0
>>> [  143.206444]  ? zap_vma_ptes+0x70/0x70
>>> [  143.206524]  ? __pm_runtime_resume+0x67/0xa0
>>> [  143.206723]  i915_gem_release_mmap+0x1ba/0x1c0 [i915]
>>> [  143.206846]  i915_vma_unbind+0x5c2/0x690 [i915]
>>> [  143.206925]  ? __lock_is_held+0x52/0x100
>>> [  143.207076]  i915_gem_object_set_tiling+0x1db/0x650 [i915]
>>> [  143.207236]  i915_gem_set_tiling_ioctl+0x1d3/0x3b0 [i915]
>>> [  143.207377]  ? i915_gem_set_tiling_ioctl+0x5/0x3b0 [i915]
>>> [  143.207457]  drm_ioctl+0x36c/0x670
>>> [  143.207535]  ? debug_lockdep_rcu_enabled.part.0+0x1a/0x30
>>> [  143.207730]  ? i915_gem_object_set_tiling+0x650/0x650 [i915]
>>> [  143.207793]  ? drm_getunique+0x120/0x120
>>> [  143.207875]  ? __handle_mm_fault+0x996/0x14a0
>>> [  143.207939]  ? vm_insert_page+0x340/0x340
>>> [  143.208028]  ? up_write+0x28/0x50
>>> [  143.208086]  ? vm_mmap_pgoff+0x160/0x190
>>> [  143.208163]  do_vfs_ioctl+0x12c/0xa60
>>> [  143.208218]  ? debug_lockdep_rcu_enabled+0x35/0x40
>>> [  143.208267]  ? ioctl_preallocate+0x150/0x150
>>> [  143.208353]  ? __do_page_fault+0x36a/0x6e0
>>> [  143.208400]  ? mark_held_locks+0x23/0xc0
>>> [  143.208479]  ? up_read+0x1f/0x40
>>> [  143.208526]  ? entry_SYSCALL_64_fastpath+0x5/0xc6
>>> [  143.208669]  ? __fget_light+0xa7/0xc0
>>> [  143.208747]  SyS_ioctl+0x41/0x70
>>>
>>> To prevent the possibility of a deadlock, we defer scheduling the worker
>>> until after we have proven that given the current mm, the userptr range
>>> does not overlap a GGTT mmaping. If another thread tries to remap the
>>> GGTT over the userptr before the worker is scheduled, it will be stopped
>>> by its invalidate-range flushing the current work, before the deadlock
>>> can occur.
>>>
>>> v2: Improve discussion of how we end up in the deadlock.
>>>
>>> Reported-by: Michał Winiarski <michal.winiarski at intel.com>
>>> Testcase: igt/gem_userptr_blits/map-fixed-invalidate-gup
>>> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
>>> Cc: Michał Winiarski <michal.winiarski at intel.com>
>>> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>> ---
>>> drivers/gpu/drm/i915/i915_gem_userptr.c | 76 ++++++++++++++++++++++-----------
>>> 1 file changed, 52 insertions(+), 24 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
>>> index dc9bf5282071..7addbf08bcb9 100644
>>> --- a/drivers/gpu/drm/i915/i915_gem_userptr.c
>>> +++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
>>> @@ -488,6 +488,36 @@ __i915_gem_userptr_set_active(struct drm_i915_gem_object *obj,
>>> 	return ret;
>>> }
>>>
>>> +static bool overlaps_ggtt(struct drm_i915_gem_object *obj, struct mm_struct *mm)
>>> +{
>>> +	const struct vm_operations_struct *gem_vm_ops =
>>> +		obj->base.dev->driver->gem_vm_ops;
>>> +	unsigned long addr = obj->userptr.ptr;
>>> +	const unsigned long end = addr + obj->base.size;
>>> +	struct vm_area_struct *vma;
>>> +
>>> +	/* Check for a contiguous set of vma covering the userptr, if any
>>> +	 * are absent, they will EFAULT. More importantly if any point back
>>> +	 * to a drm_i915_gem_object GTT mmaping, we may trigger a deadlock
>>> +	 * between the deferred gup of this userptr and the object being
>>> +	 * unbound calling invalidate_range -> cancel_userptr.
>>> +	 */
>>> +	for (vma = find_vma(mm, addr); vma; vma = vma->vm_next) {
>>> +		if (vma->vm_start > addr) /* gap */
>>> +			break;
>>
>> If I understand this correctly it is checking for more than ggtt, so
>> would be tempted to either remove those parts of the checks or
>> remove the helper to something like invalid_backing_store.
>>
>> Also, again if I understand it correctly, if the backing store was
>> made out of two VMAs then wouldn't this return a false positive?
>
> No. The vmas have to be contiguous or else the range covers an absent
> page (which will EFAULT).

I assume you imply there cannot be two adjacent VMAs, that the core 
would merge them? But I am thinking there could be two non-mergeable 
ones if the backing store was different. Not 100% sure, this is just my 
intuition talking. But it would still be a valid userptr object, one 
half backed with anon pages, another say file backed.

Regards,

Tvrtko