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Tsirkin" To: Kevin Loughlin Cc: David Hildenbrand , Jason Wang , Eugenio =?iso-8859-1?Q?P=E9rez?= , Xuan Zhuo , Ard Biesheuvel , Kiryl Shutsemau , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Tom Lendacky , Ashish Kalra , Michael Roth , Rick Edgecombe , Andrew Morton , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , Alexander Duyck , virtualization@lists.linux.dev, linux-coco@lists.linux.dev, linux-efi@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Sidharth Telang , Punit Salian , Peter Gonda , Ackerley Tng , Jacky Li , Steve Rutherford , David Skidmore Subject: Re: [PATCH 00/13] virtio_balloon, efi/unaccepted: support memory ballooning and free page reporting in CoCo guests Message-ID: <20261027163814-mutt-send-email-mst@kernel.org> References: <20261027025616.1632690-1-kevinloughlin@google.com> <20261027051245-mutt-send-email-mst@kernel.org> X-Mailing-List: linux-coco@lists.linux.dev List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 In-Reply-To: <20261027051245-mutt-send-email-mst@kernel.org> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: UouH1lCyKXlEsf4QXaTV3y0Eb_Uv-SDmHthGWwxJm-I_1793133569 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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 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 Acked-by: Konstantin Khlebnikov Acked-by: Andy Lutomirski Cc: Pavel Emelyanov Cc: Andrew Morton Cc: Mark Seaborn Cc: stable@vger.kernel.org Signed-off-by: Linus Torvalds Leaking phys addresses to unpriveleged entities might not be wise. -- MST