On 3/20/24 08:23, Peter Krempa wrote:
On Wed, Mar 20, 2024 at 10:19:14 +0100, Andrea Bolognani wrote:
Up until this point, we have avoided setting labels for
incoming migration when the TPM state is stored on a shared
filesystem. This seems to make sense, because since the
underlying storage is shared surely the labels will be as
well.
There's one problem, though: when a guest is migrated, the
SELinux context for the destination process is different from
the one of the source process.
We haven't hit any issues with the current approach so far
because NFS doesn't support SELinux, so effectively it doesn't
matter whether relabeling happens or not: even if the SELinux
contexts of the source and target processes are different,
both will be able to access the storage.
Now that it's possible for the local admin to manually mark
exported directories as shared filesystems, however, things
can get problematic.
Consider the case in which one host (mig-one) exports its
local filesystem /srv/nfs/libvirt/swtpm via NFS, and at the
same time bind-mounts it to /var/lib/libvirt/swtpm; another
host (mig-two) mounts the same filesystem to the same
location, this time via NFS. Additionally, in order to
allow migration in both directions, on mig-one the
/var/lib/libvirt/swtpm directory is listed in the
shared_filesystems qemu.conf option.
When migrating from mig-one to mig-two, things work just fine;
going in the opposite direction, however, results in an error:
# virsh migrate cirros qemu+ssh://mig-one/system
error: internal error: QEMU unexpectedly closed the monitor (vm='cirros'):
qemu-system-x86_64: tpm-emulator: Setting the stateblob (type 1) failed with a TPM error 0x1f
qemu-system-x86_64: error while loading state for instance 0x0 of device 'tpm-emulator'
qemu-system-x86_64: load of migration failed: Input/output error
This is because the directory on mig-one is considered a
shared filesystem and thus labeling is skipped, resulting in
a SELinux denial.
The solution is quite simple: remove the check and always
relabel. We know that it's okay to do so not just because it
makes the error seen above go away, but also because no such
check currently exists for disks and other types of persistent
storage such as NVRAM files, which always get relabeled.
Did you consider the case when the migration fails and the VM will be
restored to run on the source host again? In such case doin the
relabelling might break the source host.
Right. I seem to remember testing such scenarios. I had to put an exit()
(or something like it) into swtpm on the destination side to trigger the
fallback to the source side. The swtpm on the source side had closed
file access and wants to open them (lockfile) again and so the files
needed to be labeled correctly if the storage on the source side is
on the disk and exported via NFS from there (iirc). If the storage is
NFS-exported from a 3rd host it probably would not require the labels.
Stefan
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