Re: [PATCH v3 3/5] xen/livepatch: do Xen build-id check earlier

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 26/09/2024 11:14 am, Roger Pau Monne wrote:
> The check against the expected Xen build ID should be done ahead of attempting
> to apply the alternatives contained in the livepatch.
>
> If the CPUID in the alternatives patching data is out of the scope of the
> running Xen featureset the BUG() in _apply_alternatives() will trigger thus
> bringing the system down.  Note the layout of struct alt_instr could also
> change between versions.  It's also possible for struct exception_table_entry
> to have changed format, hence leading to other kind of errors if parsing of 
> the
> payload is done ahead of checking if the Xen build-id matches.
>
> Move the Xen build ID check as early as possible.  To do so introduce a new
> check_xen_buildid() function that parses and checks the Xen build-id before
> moving the payload.  Since the expected Xen build-id is used early to
> detect whether the livepatch payload could be loaded, there's no reason to
> store it in the payload struct, as a non-matching Xen build-id won't get the
> payload populated in the first place.
>
> Note printing the expected Xen build ID has part of dumping the payload
> information is no longer done: all loaded payloads would have Xen build IDs
> matching the running Xen, otherwise they would have failed to load.
>
> Fixes: 879615f5db1d ('livepatch: Always check hypervisor build ID upon 
> livepatch upload')
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>