drm/i915/gem: Prevent NULL pointer dereference on missing ctx->vm

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@canonical.com>
Fixes: 5dbd2b7be61e ("drm/i915/gem: Convert vm idr to xarray")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200123152602.1432282-1-chris@chris-wilson.co.uk

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
index 5d4157e..a67c2e1 100644 (file)
--- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
@@ -1002,13 +1002,13 @@ 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;