[PATCH v2] 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 __intel_context_retire() with active->mutex (i.e., 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>
---
v2: Reduce the scope of the mutex lock to only __intel_context_retire()
    and mark it as a function that may sleep so it doesn't run in
    atomic context

 drivers/gpu/drm/i915/gt/intel_context.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
index 57e8a051ddc2..9b9be8058881 100644
--- a/drivers/gpu/drm/i915/gt/intel_context.c
+++ b/drivers/gpu/drm/i915/gt/intel_context.c
@@ -221,6 +221,7 @@ static void __intel_context_retire(struct i915_active *active)
 
 	CE_TRACE(ce, "retire\n");
 
+	mutex_lock(&active->mutex);
 	set_bit(CONTEXT_VALID_BIT, &ce->flags);
 	if (ce->state)
 		__context_unpin_state(ce->state);
@@ -229,6 +230,7 @@ static void __intel_context_retire(struct i915_active *active)
 	__ring_retire(ce->ring);
 
 	intel_context_put(ce);
+	mutex_unlock(&active->mutex);
 }
 
 static int __intel_context_active(struct i915_active *active)
@@ -288,7 +290,8 @@ intel_context_init(struct intel_context *ce,
 	mutex_init(&ce->pin_mutex);
 
 	i915_active_init(&ce->active,
-			 __intel_context_active, __intel_context_retire);
+			 __intel_context_active,
+			 i915_active_may_sleep(__intel_context_retire));
 }
 
 void intel_context_fini(struct intel_context *ce)
-- 
2.26.0