CVE-2024-53167

In the Linux kernel, the following vulnerability has been resolved:

nfs/blocklayout: Don't attempt unregister for invalid block device

Since commit d869da91cccb ("nfs/blocklayout: Fix premature PR key unregistration") an unmount of a pNFS SCSI layout-enabled NFS may dereference a NULL block_device in:

blunregisterscsi+0x16/0xe0 [blocklayoutdriver] blfreedevice+0x70/0x80 [blocklayoutdriver] blfreedeviceidnode+0x12/0x30 [blocklayoutdriver] nfs4putdeviceidnode+0x60/0xc0 [nfsv4] nfs4deviceidpurgeclient+0x132/0x190 [nfsv4] unsetpnfslayoutdriver+0x59/0x60 [nfsv4] nfs4destroyserver+0x36/0x70 [nfsv4] nfsfreeserver+0x23/0xe0 [nfs] deactivatelockedsuper+0x30/0xb0 cleanupmnt+0xba/0x150 taskworkrun+0x59/0x90 syscallexittousermode+0x217/0x220 dosyscall64+0x8e/0x160

This happens because even though we were able to create the nfs4deviceidnode, the lookup for the device was unable to attach the block device to the pnfsblockdev.

If we never found a block device to register, we can avoid this case with the PNFSBDEVREGISTERED flag. Move the deref behind the test for the flag.