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h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Hd5V7b2lJCRJq/Ms8s/d8tR11f/qhMBk7Oe4j58RXcp6qt6aQwikHkHXDqQsG6y9I WrsMl87aoacM9e9KjV+HavYIIvBZEd/CGC6r/SsKz0KRi08xCTqHV7X/fKTG0AIXAW zPWUJE30Iut++MHgK+DBrex+Gg1pDcKlkP4p7uMQ= Date: Wed, 23 Sep 2026 11:40:36 +0100 From: Catalin Marinas To: "Aneesh Kumar K.V" Cc: linux-coco@lists.linux.dev, kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, iommu@lists.linux.dev, Andrew Morton , christian.koenig@amd.com, Jason Gunthorpe , Joerg Roedel , Marc Zyngier , Marek Szyprowski , Robin Murphy , Steven Price , Sumit Semwal , Suzuki K Poulose , Thomas Gleixner , Will Deacon , dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, linux-media@vger.kernel.org, linux-mm@kvack.org Subject: Re: [RFC PATCH v7 02/13] mm: Add an allocator for CoCo shared memory Message-ID: References: <20260921144847.501151-1-aneesh.kumar@kernel.org> <20260921144847.501151-3-aneesh.kumar@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 9F8CD180002 X-Rspam-User: X-Stat-Signature: m9hzmygyapph64yhp1oiyemt1a8nt45j X-HE-Tag: 1790160043-322699 X-HE-Meta: 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 w/EFkPsi ZM/lyRNYUSM+7O7WZGZCmRN0iORbn/feWTH0afgYB+F7rlmllS+yhJZYGuwZM2j65EKzXq1vWKOAfbTD9aIIvPPd/pIrpPoUwa1MdL96RmAtoZTOLM9JSWoVxWmdUJtBTMmmB0PXVNOKTEFI18w0AIrCoc+9TVKfxOxI2tKWmwGo1vzRdeBqHW7ok3XONS6QrAA+fTBadIQ/yVqjtC9ytkjMWBc3R18v4NZBo+ghcf/O6R1f4A1JhYNiN2XNuNw89KRZCZffMNzztFOerqyKt9K+PI7cbIw4RWEX9 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Sep 23, 2026 at 03:58:25PM +0530, Aneesh Kumar K.V wrote: > Catalin Marinas writes: > > On Wed, Sep 23, 2026 at 11:23:27AM +0530, Aneesh Kumar K.V wrote: > >> Catalin Marinas writes: > >> > On Mon, Sep 21, 2026 at 08:18:36PM +0530, Aneesh Kumar K.V (Arm) wrote: > >> >> +int alloc_cc_shared_pages_node(int nid, gfp_t gfp, > >> >> + size_t requested, struct cc_shared_pages *mem) > >> >> +{ > >> >> + struct cc_shared_layout layout; > >> >> + struct page *page; > >> >> + unsigned int order; > >> >> + bool zero = gfp & __GFP_ZERO; > >> >> + int ret; > >> >> + > >> >> + if (!mem) > >> >> + return -EINVAL; > >> >> + > >> >> + ret = cc_shared_calc_layout(requested, &layout); > >> >> + if (ret) > >> >> + return ret; > >> >> + > >> >> + order = get_order(layout.shared_size); > >> >> + if (order > MAX_PAGE_ORDER) > >> >> + return -EINVAL; > >> >> + > >> >> + /* > >> >> + * State transitions require a linear-map address and may modify memory. > >> >> + * Allocate from low memory and defer requested zeroing until afterwards. > >> >> + */ > >> >> + gfp &= ~(__GFP_HIGHMEM | __GFP_ZERO); > >> >> + if (nid == NUMA_NO_NODE) > >> >> + page = alloc_pages(gfp, order); > >> >> + else > >> >> + page = alloc_pages_node(nid, gfp, order); > >> >> + if (!page) > >> >> + return -ENOMEM; > >> >> + > >> >> + ret = cc_make_shared(page_address(page), layout.shared_size); > >> >> + if (ret) { > >> >> + if (!cc_make_private(page_address(page), layout.shared_size)) > >> >> + __free_pages(page, order); > >> >> + else > >> >> + pr_warn_ratelimited("leaking %zu bytes with uncertain shared state\n", > >> >> + layout.shared_size); > >> >> + return ret; > >> >> + } > >> >> + > >> >> + if (zero) > >> >> + memset(page_address(page), 0, layout.shared_size); > >> > > >> > Does the memset() post sharing logic work for pKVM as well? If nothing > >> > clears it, we have a small window where guest data is leaked to the > >> > host. > >> > > >> > Is there a case where we *do not* need the memory cleared? If not, maybe > >> > we can move the logic in the arch set_memory_decrypted(). > >> > > >> > >> I don't think every architecture or platform can unconditionally zero > >> memory in set_memory_decrypted(). Some callers may need to share valid > >> contents with the host. > > > > Is there any? That would be a bad assumptions in the caller. Most > > set_memory_* backends don't preserve the content as they change the > > encryption key. So properly written code shouldn't rely on this unless > > it knows specifically it's only running on pKVM for example. The only > > use-case I see to avoid explicit zeroing is when the caller doesn't care > > about the page initialisation and wants to save some cycles. The > > encryption key change would take care of the security aspect. > > > > I checked this, and you are right. We cannot expect the contents to > remain valid across sharing; set_memory_decrypted() is destructive in > that sense. Since pKVM does not rely on memory encryption, it needs to > zero the memory unconditionally in set_memory_decrypted() to avoid > exposing existing guest data. Other CoCo implementations may omit the > memset(0). We still need to zero the memory when the caller requests > __GFP_ZERO, where the memory location is expected to be zero. > > We could either zero the memory unconditionally or pass a flag to allow > this micro-optimization. We would also need to audit all call paths to > avoid redundant zeroing after set_memory_decrypted(). Let me know if you > have a preference for either approach. The simplest is probably to always zero in the backend and ignore __GFP_ZERO to the allocator. But it's probably only marginally smaller than passing a CC_SHARED_ZERO flag down. Get codex to try this as well and compare the diffstat. There's an argument for the flag approach from a performance perspective (avoid zeroing unnecessarily) but not sure how much it matters in practice. -- Catalin