Re: [PATCH 00/13] virtio_balloon, efi/unaccepted: support memory ballooning and free page reporting in CoCo guests

[Michael S. Tsirkin]

On Tue, Oct 27, 2026 at 06:29:35AM -0400, Michael S. Tsirkin wrote:
> On Tue, Oct 27, 2026 at 02:56:03AM +0000, Kevin Loughlin wrote:
> > This series enables virtio-balloon free page reporting and memory balloon
> > inflation/deflation in Confidential Computing (CoCo) guests, starting
> > with architectural hooks implemented for AMD SEV variants, in order to
> > support dynamic CoCo guest resizing.
> > 
> > Today, memory ballooning and free page reporting are essentially unusable
> > in CoCo guests due to three main roadblocks:
> 
> 
> For me, the bigger question is not whether we can, but whether we
> should. Considerations I would like to see addressed:
> 
> Security, e.g.: does knowing which memory is free leak information from
> the guest? If no, why not?
> Consider CONFIG_SHUFFLE_PAGE_ALLOCATOR, RANDSTRUCT, etc.
> 
> correctness, e.g.:  Linux has init on free (and default on windows), if
> initialized pages are dropped and host can not be trusted, they have to be
> re-initialized. Is this handled? I don't see it in code.
> 
> Also, performance, e.g.: How much does this re-initialization cost?
> 
> Management: it was simple with balloon off unconditionally.
> Now we are in murkier waters with various features offering
> various performance/security tradeoffs.
> What if some guests want the tradeoffs but others don't?
> 
> Documentation: for all of the above.
> 
> -- 
> MST

And to give this a bit more perspective, I present:

commit ab676b7d6fbf4b294bf198fb27ade5b0e865c7ce
Author: Kiryl Shutsemau <kas@xxxxxxxxxx>
Date:   Mon Mar 9 23:11:12 2015 +0200

    pagemap: do not leak physical addresses to non-privileged userspace
    
    As pointed by recent post[1] on exploiting DRAM physical imperfection,
    /proc/PID/pagemap exposes sensitive information which can be used to do
    attacks.
    
    This disallows anybody without CAP_SYS_ADMIN to read the pagemap.
    
    [1] http://googleprojectzero.blogspot.com/2015/03/exploiting-dram-rowhammer-bug-to-gain.html
    
    [ Eventually we might want to do anything more finegrained, but for now
      this is the simple model.   - Linus ]
    
    Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
    Acked-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxx>
    Acked-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
    Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
    Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
    Cc: Mark Seaborn <mseaborn@xxxxxxxxxxxx>
    Cc: stable@xxxxxxxxxxxxxxx
    Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>


Leaking phys addresses to unpriveleged entities might not be wise.

-- 
MST