[Intel-gfx] [PATCH] drm/i915/gem: Prevent NULL pointer dereference on missing ctx->vm

[Chris Wilson]

If the ctx->vm is freed before we can acquire a local reference to it,
we proceed to call i915_vm_put(NULL), which is invalid.

Reported-by: Colin Ian King <colin.king at canonical.com>
Fixes: 5dbd2b7be61e ("drm/i915/gem: Convert vm idr to xarray")
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Colin Ian King <colin.king at canonical.com>
---
 drivers/gpu/drm/i915/gem/i915_gem_context.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
index 5d4157e1ccf7..8873db5be123 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
@@ -1002,13 +1002,14 @@ static int get_ppgtt(struct drm_i915_file_private *file_priv,
 	if (!rcu_access_pointer(ctx->vm))
 		return -ENODEV;
 
-	err = -ENODEV;
 	rcu_read_lock();
 	vm = context_get_vm_rcu(ctx);
-	if (vm)
-		err = xa_alloc(&file_priv->vm_xa, &id, vm,
-			       xa_limit_32b, GFP_KERNEL);
 	rcu_read_unlock();
+	if (!vm)
+		return -ENODEV;
+
+	err = xa_alloc(&file_priv->vm_xa, &id, vm,
+		       xa_limit_32b, GFP_KERNEL);
 	if (err)
 		goto err_put;
 
-- 
2.25.0