Re: [PATCH 2/2] liveupdate: kho: allocate metadata directly from the buddy allocator

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

On Fri, Oct 24, 2025 at 10:55 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Fri, Oct 24, 2025 at 10:36:45AM -0400, Pasha Tatashin wrote:
>
> > We do not zero memory on kexec/KHO/LU; instead, the next kernel zeroes
> > memory on demand during allocation. My point is that the KHO interface
> > retrieves a full page in the next kernel, not an individual slab.
> > Consequently, a caller might retrieve data that was preserved as a
> > slab in the previous kernel, expose that data to the user, and
> > unintentionally leak the remaining part of the page as well.
>
> I don't think preventing that is part of the kho threat model..
>
> >
> > > > There's also the inefficiency. The unpreserved parts of that page are
> > > > unusable by the new kernel until the preserved object is freed.
> > >
> > > Thats not how I see slab preservation working. When the slab page
> > > is unpreserved all the free space in that page should be immediately
> > > available to the sucessor kernel.
> >
> > This ties into the same problem. The scenario I'm worried about is:
> > 1. A caller preserves one small slab object.
> > 2. In the new kernel, the caller retrieves the entire page that
> > contains this object.
> > 3. The caller uses the data from that slab object without freeing it.
>
> 4. When slab restores the page it immediately makes all the free slots
>   available on its free list.

Right, we do not have this functionality.

>
> > > other patches are small and allocating a whole page is pretty wasteful
> > > too.
> >
> > If we're going to support this, it would have to be specifically
> > engineered as full slab support for KHO preservation, where the
> > interface retrieves slab objects directly, not the pages they're on,
>
> Yes
>
> > and I think would require using a special GFP_PRESERVED flag.
>
> Maybe so, I was hoping not..
>
> Jason