[PATCH v7 1/3] drm/i915/vma: Fix UAF on destroy against retire race

[Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>]

Object debugging tools were sporadically reporting illegal attempts to
free a still active i915 VMA object when parking a GT believed to be idle.

[161.359441] ODEBUG: free active (active state 0) object: ffff88811643b958 object type: i915_active hint: __i915_vma_active+0x0/0x50 [i915]
[161.360082] WARNING: CPU: 5 PID: 276 at lib/debugobjects.c:514 debug_print_object+0x80/0xb0
...
[161.360304] CPU: 5 PID: 276 Comm: kworker/5:2 Not tainted 6.5.0-rc1-CI_DRM_13375-g003f860e5577+ #1
[161.360314] Hardware name: Intel Corporation Rocket Lake Client Platform/RocketLake S UDIMM 6L RVP, BIOS RKLSFWI1.R00.3173.A03.2204210138 04/21/2022
[161.360322] Workqueue: i915-unordered __intel_wakeref_put_work [i915]
[161.360592] RIP: 0010:debug_print_object+0x80/0xb0
...
[161.361347] debug_object_free+0xeb/0x110
[161.361362] i915_active_fini+0x14/0x130 [i915]
[161.361866] release_references+0xfe/0x1f0 [i915]
[161.362543] i915_vma_parked+0x1db/0x380 [i915]
[161.363129] __gt_park+0x121/0x230 [i915]
[161.363515] ____intel_wakeref_put_last+0x1f/0x70 [i915]

That has been tracked down to be happening when another thread is
deactivating the VMA inside __active_retire() helper, after the VMA's
active counter has been already decremented to 0, but before deactivation
of the VMA's object is reported to the object debugging tool.

We could prevent from that race by serializing i915_active_fini() with
__active_retire() via ref->tree_lock, but that wouldn't stop the VMA from
being used, e.g. from __i915_vma_retire() called at the end of
__active_retire(), after that VMA has been already freed by a concurrent
i915_vma_destroy() on return from the i915_active_fini().  Then, we should
rather fix the issue at the VMA level, not in i915_active.

Since __i915_vma_parked() is called from __gt_park() on last put of the
GT's wakeref, the issue could be addressed by holding the GT wakeref long
enough for __active_retire() to complete before that wakeref is released
and the GT parked.

I believe the issue was introduced by commit d93939730347 ("drm/i915:
Remove the vma refcount") which moved a call to i915_active_fini() from
a dropped i915_vma_release(), called on last put of the removed VMA kref,
to i915_vma_parked() processing path called on last put of a GT wakeref.
However, its visibility to the object debugging tool was suppressed by a
bug in i915_active that was fixed two weeks later with commit e92eb246feb9
("drm/i915/active: Fix missing debug object activation").

A VMA associated with a request doesn't acquire a GT wakeref by itself.
Instead, it depends on a wakeref held directly by the request's active
intel_context for a GT associated with its VM, and indirectly on that
intel_context's engine wakeref if the engine belongs to the same GT as the
VMA's VM.  Those wakerefs are released asynchronously to VMA deactivation.

Fix the issue by getting a wakeref for the VMA's GT when activating it,
and putting that wakeref only after the VMA is deactivated.  However,
exclude global GTT from that processing path, otherwise the GPU never goes
idle.  Since __i915_vma_retire() may be called from atomic contexts, use
async variant of wakeref put.  Also, to avoid circular locking dependency,
take care of acquiring the wakeref before VM mutex when both are needed.

v7: Add inline comments with justifications for:
    - using untracked variants of intel_gt_pm_get/put() (Nirmoy),
    - using async variant of _put(),
    - not getting the wakeref in case of a global GTT,
    - always getting the first wakeref outside vm->mutex.
v6: Since __i915_vma_active/retire() callbacks are not serialized, storing
    a wakeref tracking handle inside struct i915_vma is not safe, and
    there is no other good place for that.  Use untracked variants of
    intel_gt_pm_get/put_async().
v5: Replace "tile" with "GT" across commit description (Rodrigo),
  - avoid mentioning multi-GT case in commit description (Rodrigo),
  - explain why we need to take a temporary wakeref unconditionally inside
    i915_vma_pin_ww() (Rodrigo).
v4: Refresh on top of commit 5e4e06e4087e ("drm/i915: Track gt pm
    wakerefs") (Andi),
  - for more easy backporting, split out removal of former insufficient
    workarounds and move them to separate patches (Nirmoy).
  - clean up commit message and description a bit.
v3: Identify root cause more precisely, and a commit to blame,
  - identify and drop former workarounds,
  - update commit message and description.
v2: Get the wakeref before VM mutex to avoid circular locking dependency,
  - drop questionable Fixes: tag.

Fixes: d93939730347 ("drm/i915: Remove the vma refcount")
Closes: https://gitlab.freedesktop.org/drm/intel/issues/8875
Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: stable@vger.kernel.org # v5.19+
---
 drivers/gpu/drm/i915/i915_vma.c | 50 ++++++++++++++++++++++++++++-----
 1 file changed, 43 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
index d09aad34ba37f..b70715b1411d6 100644
--- a/drivers/gpu/drm/i915/i915_vma.c
+++ b/drivers/gpu/drm/i915/i915_vma.c
@@ -34,6 +34,7 @@
 #include "gt/intel_engine.h"
 #include "gt/intel_engine_heartbeat.h"
 #include "gt/intel_gt.h"
+#include "gt/intel_gt_pm.h"
 #include "gt/intel_gt_requests.h"
 #include "gt/intel_tlb.h"
 
@@ -103,12 +104,42 @@ static inline struct i915_vma *active_to_vma(struct i915_active *ref)
 
 static int __i915_vma_active(struct i915_active *ref)
 {
-	return i915_vma_tryget(active_to_vma(ref)) ? 0 : -ENOENT;
+	struct i915_vma *vma = active_to_vma(ref);
+
+	if (!i915_vma_tryget(vma))
+		return -ENOENT;
+
+	/*
+	 * Exclude global GTT VMA from holding a GT wakeref
+	 * while active, otherwise GPU never goes idle.
+	 */
+	if (!i915_vma_is_ggtt(vma)) {
+		/*
+		 * Since we and our _retire() counterpart can be
+		 * called asynchronously, storing a wakeref tracking
+		 * handle inside struct i915_vma is not safe, and
+		 * there is no other good place for that.  Hence,
+		 * use untracked variants of intel_gt_pm_get/put().
+		 */
+		intel_gt_pm_get_untracked(vma->vm->gt);
+	}
+
+	return 0;
 }
 
 static void __i915_vma_retire(struct i915_active *ref)
 {
-	i915_vma_put(active_to_vma(ref));
+	struct i915_vma *vma = active_to_vma(ref);
+
+	if (!i915_vma_is_ggtt(vma)) {
+		/*
+		 * Since we can be called from atomic contexts,
+		 * use an async variant of intel_gt_pm_put().
+		 */
+		intel_gt_pm_put_async_untracked(vma->vm->gt);
+	}
+
+	i915_vma_put(vma);
 }
 
 static struct i915_vma *
@@ -1404,7 +1435,7 @@ int i915_vma_pin_ww(struct i915_vma *vma, struct i915_gem_ww_ctx *ww,
 	struct i915_vma_work *work = NULL;
 	struct dma_fence *moving = NULL;
 	struct i915_vma_resource *vma_res = NULL;
-	intel_wakeref_t wakeref = 0;
+	intel_wakeref_t wakeref;
 	unsigned int bound;
 	int err;
 
@@ -1424,8 +1455,14 @@ int i915_vma_pin_ww(struct i915_vma *vma, struct i915_gem_ww_ctx *ww,
 	if (err)
 		return err;
 
-	if (flags & PIN_GLOBAL)
-		wakeref = intel_runtime_pm_get(&vma->vm->i915->runtime_pm);
+	/*
+	 * In case of a global GTT, we must hold a runtime-pm wakeref
+	 * while global PTEs are updated.  In other cases, we hold
+	 * the rpm reference while the VMA is active.  Since runtime
+	 * resume may require allocations, which are forbidden inside
+	 * vm->mutex, get the first rpm wakeref outside of the mutex.
+	 */
+	wakeref = intel_runtime_pm_get(&vma->vm->i915->runtime_pm);
 
 	if (flags & vma->vm->bind_async_flags) {
 		/* lock VM */
@@ -1561,8 +1598,7 @@ int i915_vma_pin_ww(struct i915_vma *vma, struct i915_gem_ww_ctx *ww,
 	if (work)
 		dma_fence_work_commit_imm(&work->base);
 err_rpm:
-	if (wakeref)
-		intel_runtime_pm_put(&vma->vm->i915->runtime_pm, wakeref);
+	intel_runtime_pm_put(&vma->vm->i915->runtime_pm, wakeref);
 
 	if (moving)
 		dma_fence_put(moving);
-- 
2.43.0