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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Sep 26, 2024 at 05:11:19PM +0100, Ross Lagerwall wrote:
> On Thu, Sep 26, 2024 at 11:21 AM Roger Pau Monne <roger.pau@xxxxxxxxxx> 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: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> 
> Should the ELF_LIVEPATCH_XEN_DEPENDS check also be dropped from
> check_special_sections() since it is no longer necessary?

It's dropped from check_special_sections() in this patch, just not
mentioned in the commit message I'm afraid.

Thanks, Roger.