[PATCH] drm/i915: Fix the use-after-free between i915_active_ref and __active_retire

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We easily hit drm/i915 panic on TGL when running glmark2, and finally
caught the race condition of use-after-free with enabling KASAN.

The call stack is below:
===
[  534.472675] BUG: KASAN: use-after-free in __i915_active_fence_set+0x26d/0x3d0 [i915]
[  534.472679] Write of size 8 at addr ffff8883f0372388 by task glmark2/3199

[  534.472684] CPU: 3 PID: 3199 Comm: glmark2 Tainted: G     U      E     5.4.0-rc8 #8
[  534.472687] Call Trace:
[  534.472693]  dump_stack+0x95/0xd5
[  534.472722]  ? __i915_active_fence_set+0x26d/0x3d0 [i915]
[  534.472727]  print_address_description.constprop.5+0x20/0x320
[  534.472751]  ? __i915_active_fence_set+0x26d/0x3d0 [i915]
[  534.472792]  ? __i915_active_fence_set+0x26d/0x3d0 [i915]
[  534.472794]  __kasan_report+0x149/0x18c
[  534.472798]  ? _raw_spin_lock+0x1/0xd0
[  534.472820]  ? __i915_active_fence_set+0x26d/0x3d0 [i915]
[  534.472822]  kasan_report+0x12/0x20
[  534.472825]  __asan_report_store8_noabort+0x17/0x20
[  534.472847]  __i915_active_fence_set+0x26d/0x3d0 [i915]
[  534.472870]  i915_active_ref+0x2c8/0x530 [i915]
[  534.472874]  ? dma_resv_add_shared_fence+0x291/0x460
[  534.472902]  __i915_vma_move_to_active+0x56/0x70 [i915]
[  534.472927]  i915_vma_move_to_active+0x54/0x420 [i915]
[  534.472931]  ? mutex_unlock+0x22/0x40
[  534.472957]  i915_gem_do_execbuffer+0x1d45/0x3e20 [i915]
[  534.472959]  ? __kmalloc_node+0x12c/0x350
[  534.472983]  ? eb_relocate_slow+0xb40/0xb40 [i915]
[  534.472985]  ? _raw_write_trylock+0x110/0x110
[  534.472987]  ? get_partial_node.isra.72+0x51/0x260
[  534.472991]  ? unix_stream_read_generic+0x583/0x1a80
[  534.472994]  ? ___slab_alloc+0x1d8/0x550
[  534.472998]  ? kvmalloc_node+0x31/0x80
[  534.473000]  ? kasan_unpoison_shadow+0x35/0x50
[  534.473002]  ? _raw_spin_lock+0x7b/0xd0
[  534.473004]  ? radix_tree_lookup+0xd/0x10
[  534.473006]  ? idr_find+0x3b/0x60
[  534.473029]  i915_gem_execbuffer2_ioctl+0x634/0x8a0 [i915]
[  534.473052]  ? i915_gem_execbuffer_ioctl+0xd50/0xd50 [i915]
[  534.473054]  ? unix_stream_recvmsg+0x97/0xd0
[  534.473056]  ? unix_stream_splice_read+0x1c0/0x1c0
[  534.473058]  ? __unix_insert_socket+0x180/0x180
[  534.473081]  ? i915_gem_execbuffer_ioctl+0xd50/0xd50 [i915]
[  534.473094]  drm_ioctl_kernel+0x1ed/0x2b0 [drm]
[  534.473103]  ? drm_setversion+0x8c0/0x8c0 [drm]
[  534.473106]  ? __kasan_check_write+0x14/0x20
[  534.473115]  drm_ioctl+0x68b/0xaa0 [drm]
...

[  534.473239] Allocated by task 3199:
[  534.473241]  save_stack+0x21/0x90
[  534.473243]  __kasan_kmalloc.constprop.8+0xa7/0xd0
[  534.473245]  kasan_slab_alloc+0x11/0x20
[  534.473246]  kmem_cache_alloc+0xce/0x240
[  534.473273]  i915_active_ref+0xc2/0x530 [i915]
[  534.473302]  __i915_vma_move_to_active+0x56/0x70 [i915]
[  534.473328]  i915_vma_move_to_active+0x54/0x420 [i915]
[  534.473355]  i915_gem_do_execbuffer+0x1d45/0x3e20 [i915]
[  534.473381]  i915_gem_execbuffer2_ioctl+0x634/0x8a0 [i915]
[  534.473392]  drm_ioctl_kernel+0x1ed/0x2b0 [drm]
[  534.473402]  drm_ioctl+0x68b/0xaa0 [drm]
[  534.473404]  do_vfs_ioctl+0x19a/0xf10
[  534.473405]  ksys_ioctl+0x75/0x80
[  534.473407]  __x64_sys_ioctl+0x73/0xb0
[  534.473408]  do_syscall_64+0x9f/0x3a0
[  534.473410]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

[  534.473412] Freed by task 0:
[  534.473414]  save_stack+0x21/0x90
[  534.473415]  __kasan_slab_free+0x137/0x190
[  534.473417]  kasan_slab_free+0xe/0x10
[  534.473418]  kmem_cache_free+0xeb/0x2c0
[  534.473444]  __active_retire+0x1f2/0x240 [i915]
[  534.473471]  active_retire+0x13b/0x1b0 [i915]
[  534.473496]  node_retire+0x54/0x80 [i915]
[  534.473523]  intel_engine_breadcrumbs_irq+0x5f0/0xd10 [i915]
[  534.473549]  cs_irq_handler+0x66/0xb0 [i915]
[  534.473575]  gen11_gt_irq_handler+0x26c/0x400 [i915]
[  534.473600]  gen11_irq_handler+0xc3/0x250 [i915]
[  534.473603]  __handle_irq_event_percpu+0xe0/0x4c0
[  534.473605]  handle_irq_event_percpu+0x71/0x140
[  534.473606]  handle_irq_event+0xad/0x140
[  534.473608]  handle_edge_irq+0x1f6/0x780
[  534.473610]  do_IRQ+0x9f/0x1f0

[  534.473612] The buggy address belongs to the object at ffff8883f0372380
                which belongs to the cache active_node of size 72
[  534.473615] The buggy address is located 8 bytes inside of

===

The race scenerio is like:
Initially ref->count is 1, interrupt handler is trying to free the
node.

===
CPUA in interrupt context                CPUB in i915_gem_execbuffer2_ioctl
__active_retire -->
  spin_lock(&ref->tree_lock)
  decrease ref->count to 0
                                         i915_active_ref -->
                                           increase ref->count to 1
                                           (i915_active_acquire)

                                           get the dirty ref->cache
                                              (READ_ONCE(ref->cache))

                                           return the dirty node

  set ref->cache to NULL
  spin_unlock(&ref->tree_lock)
  free the node

                                           hit use-after-free in
                                              __i915_active_fence_set()

===

Here we need to use spinlock ref->tree_lock to protect the access
of READ_ONCE(ref->cache), then the race scenerio can be resolved.

with this patch, it passed our stress test for a very long time.

Signed-off-by: Chuansheng Liu <chuansheng.liu@xxxxxxxxx>
---
 drivers/gpu/drm/i915/i915_active.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
index dca15ace88f6..3d68b910e949 100644
--- a/drivers/gpu/drm/i915/i915_active.c
+++ b/drivers/gpu/drm/i915/i915_active.c
@@ -214,7 +214,10 @@ active_instance(struct i915_active *ref, struct intel_timeline *tl)
 	 * after the previous activity has been retired, or if it matches the
 	 * current timeline.
 	 */
+	spin_lock_irq(&ref->tree_lock);
 	node = READ_ONCE(ref->cache);
+	spin_unlock_irq(&ref->tree_lock);
+
 	if (node && node->timeline == idx)
 		return &node->base;
 
-- 
2.17.1

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