[PATCH] drm/i915: Fix use-after-free due to intel_context_pin/unpin race

[Sultan Alsawaf <sultan@kerneltoast.com>]

From: Sultan Alsawaf <sultan@kerneltoast.com>

The retire and active callbacks can run simultaneously, allowing
intel_context_pin() and intel_context_unpin() to run at the same time,
trashing the ring and page tables. In 5.4, this was more noticeable
because intel_ring_unpin() would set ring->vaddr to NULL and cause a
clean NULL-pointer-dereference panic, but in newer kernels the
use-after-free goes unnoticed.

The NULL-pointer-dereference looks like this:
BUG: unable to handle page fault for address: 0000000000003448
RIP: 0010:gen8_emit_flush_render+0x163/0x190
Call Trace:
 execlists_request_alloc+0x25/0x40
 __i915_request_create+0x1f4/0x2c0
 i915_request_create+0x71/0xc0
 i915_gem_do_execbuffer+0xb98/0x1a80
 ? preempt_count_add+0x68/0xa0
 ? _raw_spin_lock+0x13/0x30
 ? _raw_spin_unlock+0x16/0x30
 i915_gem_execbuffer2_ioctl+0x1de/0x3c0
 ? i915_gem_busy_ioctl+0x7f/0x1d0
 ? i915_gem_execbuffer_ioctl+0x2d0/0x2d0
 drm_ioctl_kernel+0xb2/0x100
 drm_ioctl+0x209/0x360
 ? i915_gem_execbuffer_ioctl+0x2d0/0x2d0
 ksys_ioctl+0x87/0xc0
 __x64_sys_ioctl+0x16/0x20
 do_syscall_64+0x4e/0x150
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Protect the retire callback with ref->mutex to complement the active
callback and fix the corruption.

Fixes: 12c255b5dad1 ("drm/i915: Provide an i915_active.acquire callback")
Cc: <stable@vger.kernel.org>
Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
---
 drivers/gpu/drm/i915/i915_active.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
index c4048628188a..0478bcf061b5 100644
--- a/drivers/gpu/drm/i915/i915_active.c
+++ b/drivers/gpu/drm/i915/i915_active.c
@@ -148,8 +148,10 @@ __active_retire(struct i915_active *ref)
 	spin_unlock_irqrestore(&ref->tree_lock, flags);
 
 	/* After the final retire, the entire struct may be freed */
+	mutex_lock(&ref->mutex);
 	if (ref->retire)
 		ref->retire(ref);
+	mutex_unlock(&ref->mutex);
 
 	/* ... except if you wait on it, you must manage your own references! */
 	wake_up_var(ref);
-- 
2.26.0