[PATCH][next] drm/i915/gem: fix null pointer dereference on vm

[Chris Wilson]

Quoting Colin King (2020-01-23 15:14:06)
> From: Colin Ian King <colin.king at canonical.com>
> 
> Currently if the call to function context_get_vm_rcu returns
> a null pointer for vm then the error exit path via label err_put
> will call i915_vm_put on the null vm, causing a null pointer
> dereference.  Fix this by adding a null check on vm and returning
> without calling the i915_vm_put.
> 
> Fixes: 5dbd2b7be61e ("drm/i915/gem: Convert vm idr to xarray")
> Signed-off-by: Colin Ian King <colin.king at canonical.com>

Hmm. Actually, we can drop the rcu_read_lock as soon as we've acquire
the local ref to ctx->vm. So something like,

        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;

would work.
-Chris