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

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Message-ID: <20251024145508.GD760669@ziepe.ca>
Date: Fri, 24 Oct 2025 11:55:08 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: Pratyush Yadav <pratyush@...nel.org>, akpm@...ux-foundation.org,
	brauner@...nel.org, corbet@....net, graf@...zon.com,
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
	linux-mm@...ck.org, masahiroy@...nel.org, ojeda@...nel.org,
	rdunlap@...radead.org, rppt@...nel.org, tj@...nel.org,
	jasonmiu@...gle.com, dmatlack@...gle.com, skhawaja@...gle.com,
	glider@...gle.com, elver@...gle.com
Subject: Re: [PATCH 2/2] liveupdate: kho: allocate metadata directly from the
 buddy allocator

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.

> > 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