drm/i915/gt: Check cacheline is valid before acquiring

The hwsp_cacheline pointer from i915_request is very, very flimsy. The
i915_request.timeline (and the hwsp_cacheline) are lost upon retiring
(after an RCU grace). Therefore we need to confirm that once we have the
right pointer for the cacheline, it is not in the process of being
retired and disposed of before we attempt to acquire a reference to the
cacheline.

<3>[  547.208237] BUG: KASAN: use-after-free in active_debug_hint+0x6a/0x70 [i915]
<3>[  547.208366] Read of size 8 at addr ffff88822a0d2710 by task gem_exec_parall/2536

<4>[  547.208547] CPU: 3 PID: 2536 Comm: gem_exec_parall Tainted: G     U            5.7.0-rc2-ged7a286b5d02d-kasan_117+ #1
<4>[  547.208556] Hardware name: Dell Inc. XPS 13 9350/, BIOS 1.4.12 11/30/2016
<4>[  547.208564] Call Trace:
<4>[  547.208579]  dump_stack+0x96/0xdb
<4>[  547.208707]  ? active_debug_hint+0x6a/0x70 [i915]
<4>[  547.208719]  print_address_description.constprop.6+0x16/0x310
<4>[  547.208841]  ? active_debug_hint+0x6a/0x70 [i915]
<4>[  547.208963]  ? active_debug_hint+0x6a/0x70 [i915]
<4>[  547.208975]  __kasan_report+0x137/0x190
<4>[  547.209106]  ? active_debug_hint+0x6a/0x70 [i915]
<4>[  547.209127]  kasan_report+0x32/0x50
<4>[  547.209257]  ? i915_gemfs_fini+0x40/0x40 [i915]
<4>[  547.209376]  active_debug_hint+0x6a/0x70 [i915]
<4>[  547.209389]  debug_print_object+0xa7/0x220
<4>[  547.209405]  ? lockdep_hardirqs_on+0x348/0x5f0
<4>[  547.209426]  debug_object_assert_init+0x297/0x430
<4>[  547.209449]  ? debug_object_free+0x360/0x360
<4>[  547.209472]  ? lock_acquire+0x1ac/0x8a0
<4>[  547.209592]  ? intel_timeline_read_hwsp+0x4f/0x840 [i915]
<4>[  547.209737]  ? i915_active_acquire_if_busy+0x66/0x120 [i915]
<4>[  547.209861]  i915_active_acquire_if_busy+0x66/0x120 [i915]
<4>[  547.209990]  ? __live_alloc.isra.15+0xc0/0xc0 [i915]
<4>[  547.210005]  ? rcu_read_lock_sched_held+0xd0/0xd0
<4>[  547.210017]  ? print_usage_bug+0x580/0x580
<4>[  547.210153]  intel_timeline_read_hwsp+0xbc/0x840 [i915]
<4>[  547.210284]  __emit_semaphore_wait+0xd5/0x480 [i915]
<4>[  547.210415]  ? i915_fence_get_timeline_name+0x110/0x110 [i915]
<4>[  547.210428]  ? lockdep_hardirqs_on+0x348/0x5f0
<4>[  547.210442]  ? _raw_spin_unlock_irq+0x2a/0x40
<4>[  547.210567]  ? __await_execution.constprop.51+0x2e0/0x570 [i915]
<4>[  547.210706]  i915_request_await_dma_fence+0x8f7/0xc70 [i915]

Fixes: 85bedbf1 ("drm/i915/gt: Eliminate the trylock for reading a timeline's hwsp")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: <stable@vger.kernel.org> # v5.6+
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200427093038.29219-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_timeline.c

@@ -544,6 +544,8 @@ int intel_timeline_read_hwsp(struct i915_request *from,
 
 	rcu_read_lock();
 	cl = rcu_dereference(from->hwsp_cacheline);
+	if (i915_request_completed(from)) /* confirm cacheline is valid */
+		goto unlock;
 	if (unlikely(!i915_active_acquire_if_busy(&cl->active)))
 		goto unlock; /* seqno wrapped and completed! */
 	if (unlikely(i915_request_completed(from)))