[PATCH 1/4] KVM: arm64: vgic: Don't assume the VM debugfs directory exists

[Oliver Upton]

On Sat, Apr 02, 2022 at 05:40:41PM +0000, Oliver Upton wrote:
> Unfortunately, there is no guarantee that KVM was able to instantiate a
> debugfs directory for a particular VM. To that end, KVM shouldn't even
> attempt to create new debugfs files in this case. If the specified
> parent dentry is NULL, debugfs_create_file() will instantiate files at
> the root of debugfs.
> 
> Since it is possible to create the vgic-state file outside of a VM
> directory, the file is not cleaned up when a VM is destroyed.
> Nonetheless, the corresponding struct kvm is freed when the VM is
> destroyed.
> 
> Plug the use-after-free by plainly refusing to create vgic-state when
> KVM fails to create a VM debugfs dir.
> 
> Cc: stable at kernel.org
> Fixes: 929f45e32499 ("kvm: no need to check return value of debugfs_create functions")
> Signed-off-by: Oliver Upton <oupton at google.com>

Thinking about this a bit more...

The game of whack-a-mole for other files that possibly have the same bug
could probably be avoided if kvm->debugfs_dentry is initialized to
PTR_ERR(-ENOENT) by default. That way there's no special error handling
that needs to be done in KVM as any attempt to create a new file will
bail.

Going to test and send out a v2 most likely, just want to make sure no
other use of debugfs in KVM will flip out from the change.

--
Thanks,
Oliver