[PATCH] drm/i915: Avoid potential vm use-after-free

[Rob Clark <robdclark@gmail.com>]

From: Rob Clark <robdclark@chromium.org>

Adding the vm to the vm_xa table makes it visible to userspace, which
could try to race with us to close the vm.  So we need to take our extra
reference before putting it in the table.

Signed-off-by: Rob Clark <robdclark@chromium.org>
---
Note, you could list commit e1a7ab4fca0c ("drm/i915: Remove the vm open
count") as the "fixed" commit, but really the issue seems to go back
much further (with the fix needing some backporting in the process).

 drivers/gpu/drm/i915/gem/i915_gem_context.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
index 6250de9b9196..e4b78ab4773b 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
@@ -1861,11 +1861,19 @@ static int get_ppgtt(struct drm_i915_file_private *file_priv,
 	vm = ctx->vm;
 	GEM_BUG_ON(!vm);
 
+	/*
+	 * Get a reference for the allocated handle.  Once the handle is
+	 * visible in the vm_xa table, userspace could try to close it
+	 * from under our feet, so we need to hold the extra reference
+	 * first.
+	 */
+	i915_vm_get(vm);
+
 	err = xa_alloc(&file_priv->vm_xa, &id, vm, xa_limit_32b, GFP_KERNEL);
-	if (err)
+	if (err) {
+		i915_vm_put(vm);
 		return err;
-
-	i915_vm_get(vm);
+	}
 
 	GEM_BUG_ON(id == 0); /* reserved for invalid/unassigned ppgtt */
 	args->value = id;
-- 
2.38.1