Re: [PATCH 2/3] xen/livepatch: do Xen build-id check earlier

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 23/09/2024 12:04 pm, Andrew Cooper wrote:
> On 20/09/2024 10:36 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 possibly leading to other errors.
>>
>> Move the Xen build ID check to be done ahead of any processing of the 
>> livepatch
>> payload sections.
>>
>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>> ---
>>  xen/common/livepatch.c | 9 +++++----
>>  1 file changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/xen/common/livepatch.c b/xen/common/livepatch.c
>> index cea47ffe4c84..3e4fce036a1c 100644
>> --- a/xen/common/livepatch.c
>> +++ b/xen/common/livepatch.c
>> @@ -767,6 +767,11 @@ static int prepare_payload(struct payload *payload,
>>      if ( rc )
>>          return rc;
>>  
>> +    /* Perform the Xen build-id check ahead of doing any more processing. */
>> +    rc = xen_build_id_dep(payload);
>> +    if ( rc )
>> +        return rc;
>> +
> While a step in the right direction, I think this needs to be moved far
> earlier.  Even here, it's behind the processing of the livepatch func
> state, which is something that can also change like alt_instr.
>
> The buildid checks need to be as early as possible.  Looking through the
> logic (which doesn't have great names/splits), I'd say the buildid
> checks want to be between livepatch_elf_load() (which parses the
> structure of the ELF), and move_payload() (which starts copying it into
> place).
>
> That would involve moving check_special_sections() too, but I think it's
> the right thing to do.
>
> Absolutely nothing good can come from continuing to process/setup a
> livepatch identified as "not for this hypervisor".

Further, we should probably fuzz this (KFX style).  I bet the
alternatives aren't the only now-reachable BUG()'s.

While we don't expect people to pass bad livepatches, this mess was
discovered by our CI system getting confused and passing the wrong (of
several otherwise good) livepatch for the currently running hypervisor.

~Andrew