[PATCH v3] drm/i915: Fix a VMA UAF for multi-gt platform

[Nirmoy Das]

Ensure correct handling of closed VMAs on multi-gt platforms to prevent
Use-After-Free. Currently, when GT0 goes idle, closed VMAs that are
exclusively added to GT0's closed_vma link (gt->closed_vma) and
subsequently freed by i915_vma_parked(), which assumes the entire GPU is
idle. However, on platforms with multiple GTs, such as MTL, GT1 may
remain active while GT0 is idle. This causes GT0 to mistakenly consider
the closed VMAs in its closed_vma list as unnecessary, potentially
leading to Use-After-Free issues if a job for GT1 attempts to access a
freed VMA.

Although we do take a wakeref for GT0 but it happens later, after
evaluating VMAs. To mitigate this, it is necessary to hold a GT0 wakeref
early.

v2: Use gt id to detect multi-tile(Andi)
    Fix the incorrect error path.
v3: Add more comment(Andi)
    Use the new gt var when possible(Andrzej)

Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>
Cc: Chris Wilson <chris.p.wilson at intel.com>
Cc: Andi Shyti <andi.shyti at linux.intel.com>
Cc: Andrzej Hajda <andrzej.hajda at intel.com>
Cc: Sushma Venkatesh Reddy <sushma.venkatesh.reddy at intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das at intel.com>
Tested-by: Andi Shyti <andi.shyti at linux.intel.com>
Reviewed-by: Andi Shyti <andi.shyti at linux.intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda at intel.com>
---
 .../gpu/drm/i915/gem/i915_gem_execbuffer.c    | 21 +++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
index 5fb459ea4294..1de9de1e4782 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
@@ -2692,6 +2692,7 @@ static int
 eb_select_engine(struct i915_execbuffer *eb)
 {
 	struct intel_context *ce, *child;
+	struct intel_gt *gt;
 	unsigned int idx;
 	int err;
 
@@ -2715,10 +2716,17 @@ eb_select_engine(struct i915_execbuffer *eb)
 		}
 	}
 	eb->num_batches = ce->parallel.number_children + 1;
+	gt = ce->engine->gt;
 
 	for_each_child(ce, child)
 		intel_context_get(child);
-	intel_gt_pm_get(ce->engine->gt);
+	intel_gt_pm_get(gt);
+	/*
+	 * Keep GT0 active on MTL so that i915_vma_parked() doesn't
+	 * free VMAs while execbuf ioctl is validating VMAs.
+	 */
+	if (gt->info.id)
+		intel_gt_pm_get(to_gt(gt->i915));
 
 	if (!test_bit(CONTEXT_ALLOC_BIT, &ce->flags)) {
 		err = intel_context_alloc_state(ce);
@@ -2757,7 +2765,10 @@ eb_select_engine(struct i915_execbuffer *eb)
 	return err;
 
 err:
-	intel_gt_pm_put(ce->engine->gt);
+	if (gt->info.id)
+		intel_gt_pm_put(to_gt(gt->i915));
+
+	intel_gt_pm_put(gt);
 	for_each_child(ce, child)
 		intel_context_put(child);
 	intel_context_put(ce);
@@ -2770,6 +2781,12 @@ eb_put_engine(struct i915_execbuffer *eb)
 	struct intel_context *child;
 
 	i915_vm_put(eb->context->vm);
+	/*
+	 * This works in conjunction with eb_select_engine() to prevent
+	 * i915_vma_parked() from interfering while execbuf validates vmas.
+	 */
+	if (eb->gt->info.id)
+		intel_gt_pm_put(to_gt(eb->gt->i915));
 	intel_gt_pm_put(eb->gt);
 	for_each_child(eb->context, child)
 		intel_context_put(child);
-- 
2.39.0