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

[Pasha Tatashin <pasha.tatashin@soleen.com>]

On 05-06 11:02, Pratyush Yadav wrote:
> 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

That is unrelated to the provided patch series. The author of this 
series reported this as a security issue to the Linux security ML, and 
submitted this series at their request.

This is not a security issue, and in fact, it is not an issue at all. A 
restore-only variant can be added, but I do not see a reason for LUO to 
use 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.

I am not sure what bug you are looking at (please share the details!), 
but the fix absolutely should be to use outgoing/incoming sessions 
properly, and if we mixed them up somewhere, THAT should be fixed. Using 
KHO restore is not going to help much; however, I agree it can add 
some extra scrutiny (i.e., similar to an ASSERT), but it is not really 
something that would help improve correctness in any meaningful way. The 
correctness should lie in the LUO logic using incoming as incoming, and 
outgoing as outgoing.

> 
> 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