drm/i915: Avoid potential vm use-after-free

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>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Fixes: 9ec8795e7d91 ("drm/i915: Drop __rcu from gem_context->vm")
Cc: <stable@vger.kernel.org> # v5.16+
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230119173321.2825472-1-robdclark@gmail.com

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
index 42a39e103d7c123aa92c9c6342d5963936d4096f..6d639ca24dfbb71118c7ac75e2c1d11584adbebd 100644 (file)
--- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
@@ -1860,11 +1860,19 @@ 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;