drm/i915: Confirm the request is still active before adding it to the await

Although we do check the completion-status of the request before
actually adding a wait on it (either to its submit fence or its
completion dma-fence), we currently do not check before adding it to the
dependency lists.

In fact, without checking for a completed request we may try to use the
signaler after it has been retired and its dependency tree freed:

[   60.044057] BUG: KASAN: use-after-free in __list_add_valid+0x1d/0xd0 at addr ffff880348c9e6a0
[   60.044118] Read of size 8 by task gem_exec_fence/530
[   60.044164] CPU: 1 PID: 530 Comm: gem_exec_fence Tainted: G            E   4.11.0-rc7+ #46
[   60.044226] Hardware name: ��������������������������������� ���������������������������������/���������������������������������, BIOS RYBDWi35.86A.0246.2
[   60.044290] Call Trace:
[   60.044337]  dump_stack+0x4d/0x6a
[   60.044383]  kasan_object_err+0x21/0x70
[   60.044435]  kasan_report+0x225/0x4e0
[   60.044488]  ? __list_add_valid+0x1d/0xd0
[   60.044534]  ? kasan_kmalloc+0xad/0xe0
[   60.044587]  __asan_load8+0x5e/0x70
[   60.044639]  __list_add_valid+0x1d/0xd0
[   60.044788]  __i915_priotree_add_dependency+0x67/0x130 [i915]
[   60.044895]  i915_gem_request_await_request+0xa8/0x370 [i915]
[   60.044974]  i915_gem_request_await_dma_fence+0x129/0x140 [i915]
[   60.045049]  i915_gem_do_execbuffer.isra.37+0xb0a/0x26b0 [i915]
[   60.045077]  ? save_stack+0xb1/0xd0
[   60.045105]  ? save_stack_trace+0x1b/0x20
[   60.045132]  ? save_stack+0x46/0xd0
[   60.045158]  ? kasan_kmalloc+0xad/0xe0
[   60.045184]  ? __kmalloc+0xd8/0x670
[   60.045229]  ? drm_ioctl+0x359/0x640 [drm]
[   60.045256]  ? SyS_ioctl+0x41/0x70
[   60.045330]  ? i915_vma_move_to_active+0x540/0x540 [i915]
[   60.045360]  ? tty_insert_flip_string_flags+0xa1/0xf0
[   60.045387]  ? tty_flip_buffer_push+0x63/0x70
[   60.045414]  ? remove_wait_queue+0xa9/0xc0
[   60.045441]  ? kasan_unpoison_shadow+0x35/0x50
[   60.045467]  ? kasan_kmalloc+0xad/0xe0
[   60.045494]  ? kasan_check_write+0x14/0x20
[   60.045568]  i915_gem_execbuffer2+0xdb/0x2a0 [i915]
[   60.045616]  drm_ioctl+0x359/0x640 [drm]
[   60.045705]  ? i915_gem_execbuffer+0x5a0/0x5a0 [i915]
[   60.045751]  ? drm_version+0x150/0x150 [drm]
[   60.045778]  ? compat_start_thread+0x60/0x60
[   60.045805]  ? plist_del+0xda/0x1a0
[   60.045833]  do_vfs_ioctl+0x12e/0x910
[   60.045860]  ? ioctl_preallocate+0x130/0x130
[   60.045886]  ? pci_mmcfg_check_reserved+0xc0/0xc0
[   60.045913]  ? vfs_write+0x196/0x240
[   60.045939]  ? __fget_light+0xa7/0xc0
[   60.045965]  SyS_ioctl+0x41/0x70
[   60.045991]  entry_SYSCALL_64_fastpath+0x17/0x98
[   60.046017] RIP: 0033:0x7feb2baefc47
[   60.046042] RSP: 002b:00007fff56d28e58 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[   60.046075] RAX: ffffffffffffffda RBX: 00007fff56d290a8 RCX: 00007feb2baefc47
[   60.046102] RDX: 00007fff56d29050 RSI: 00000000c0406469 RDI: 0000000000000003
[   60.046129] RBP: 00007fff56d29050 R08: 000055ecc4cd27d0 R09: 00007feb2bda8600
[   60.046154] R10: 0000000000000073 R11: 0000000000000246 R12: 00000000c0406469
[   60.046177] R13: 0000000000000003 R14: 000000000000000f R15: 0000000000000099
[   60.046203] Object at ffff880348c9e680, in cache i915_dependency size: 64
[   60.046225] Allocated:
[   60.046246] PID = 530
[   60.046269]  save_stack_trace+0x1b/0x20
[   60.046292]  save_stack+0x46/0xd0
[   60.046318]  kasan_kmalloc+0xad/0xe0
[   60.046343]  kasan_slab_alloc+0x12/0x20
[   60.046368]  kmem_cache_alloc+0xab/0x650
[   60.046445]  i915_gem_request_await_request+0x88/0x370 [i915]
[   60.046559]  i915_gem_request_await_dma_fence+0x129/0x140 [i915]
[   60.046705]  i915_gem_do_execbuffer.isra.37+0xb0a/0x26b0 [i915]
[   60.046849]  i915_gem_execbuffer2+0xdb/0x2a0 [i915]
[   60.046936]  drm_ioctl+0x359/0x640 [drm]
[   60.046987]  do_vfs_ioctl+0x12e/0x910
[   60.047038]  SyS_ioctl+0x41/0x70
[   60.047090]  entry_SYSCALL_64_fastpath+0x17/0x98
[   60.047139] Freed:
[   60.047179] PID = 530
[   60.047223]  save_stack_trace+0x1b/0x20
[   60.047269]  save_stack+0x46/0xd0
[   60.047317]  kasan_slab_free+0x72/0xc0
[   60.047366]  kmem_cache_free+0x39/0x160
[   60.047512]  i915_gem_request_retire+0x83f/0x930 [i915]
[   60.047657]  i915_gem_request_alloc+0x166/0x600 [i915]
[   60.047799]  i915_gem_do_execbuffer.isra.37+0xad8/0x26b0 [i915]
[   60.047897]  i915_gem_execbuffer2+0xdb/0x2a0 [i915]
[   60.047942]  drm_ioctl+0x359/0x640 [drm]
[   60.047968]  do_vfs_ioctl+0x12e/0x910
[   60.047993]  SyS_ioctl+0x41/0x70
[   60.048019]  entry_SYSCALL_64_fastpath+0x17/0x98
[   60.048044] Memory state around the buggy address:
[   60.048066]  ffff880348c9e580: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
[   60.048105]  ffff880348c9e600: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
[   60.048138] >ffff880348c9e680: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
[   60.048170]                                ^
[   60.048191]  ffff880348c9e700: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
[   60.048225]  ffff880348c9e780: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc

Note to hit the use-after-free requires us to be passed back a request
via a fence-array, that is from explicit fencing accumulated into a
sync-file fence-array.

Fixes: 52e54209 ("drm/i915/scheduler: Record all dependencies upon request construction")
Testcase: igt/gem_exec_fence/expired-history
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170422081537.6468-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_request.c

@@ -680,6 +680,9 @@ i915_gem_request_await_request(struct drm_i915_gem_request *to,
 
 	GEM_BUG_ON(to == from);
 
+	if (i915_gem_request_completed(from))
+		return 0;
+
 	if (to->engine->schedule) {
 		ret = i915_priotree_add_dependency(to->i915,
 						   &to->priotree,