Re: [PATCH v2 0/5] liveupdate: validate restored LUO metadata

[Pratyush Yadav <pratyush@kernel.org>]

Hi Pasha,

On Fri, May 01 2026, Pasha Tatashin wrote:

> On 05-02 01:30, Cris Jacob Maamor wrote:
>> LUO restores metadata from KHO/FDT during liveupdate. The restored
>> metadata contains physical addresses and count fields used to access and
>> walk preserved session, file set, and FLB arrays.
>> 
>> This series adds a non-consuming KHO preserved-range check and uses it
>> before phys_to_virt() on restored metadata addresses. It also rejects
>> restored counts above LUO_SESSION_MAX, LUO_FILE_MAX, and LUO_FLB_MAX
>> before traversal.
>> 
>> As far as I can tell, this is root/admin-only; I do not have evidence
>> that a normal unprivileged user can trigger it directly.
>> 
>> Changes since v1:
>> - Dropped RFC marking.
>> - Added changelog text to each patch.
>> - No code changes.
>> 
>> Cris Jacob Maamor (5):
>>   kexec: handover: add helper to check preserved page ranges
>>   liveupdate: validate LUO FDT physical address before mapping
>>   liveupdate: validate restored LUO session metadata
>>   liveupdate: validate restored LUO file set metadata
>>   liveupdate: validate restored LUO FLB metadata
>
> I have replied separately in the security report to clarify that this is 
> not a bug. The behavior follows the ABI specification exactly: we use 
> the PA addresses and ranges provided by the KHO FDT tree.
>
> NAK

I really do think we should do a restore-only variant for the
kho_alloc_preserve() family of allocators and use it everywhere. It
would prevent problems in the future. Not because the previous kernel is
malicious, but because we might have bugs and the KHO page magic sanity
check acts as a defense in depth.

For example, I am currently looking at a LUO bug where LUO does not
track if a session is outgoing or incoming. So you can do a retrieve()
or finish() on an outgoing session. A lot of nastiness is saved because
of the page magic check. Things like kho_restore_vmalloc() or
kho_restore_folio() fail early and loudly.

If we want to squeeze out more performance later down the line we can
move it behind a debug config, but having this usage pattern of always
restoring before using is going to be a lot more sane than just using
physical addresses willy nilly.

The approach this series takes with kho_is_preserved() is the wrong
design. But a kho_restore() or something similar (maybe we can find a
better name?) is really where we should be going.

-- 
Regards,
Pratyush Yadav