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b=dfiMSTdwFtu0GCkhWemNFDgm+eZPZTNwX9NfJANQl2p7VUL12gPuLOT5GFrp9s1/W gYHsfaLK7hikvujYiuXBUn8tMKa90RN/oaYf2S0NAbKARN3nHId0g1n9dBJrUeJeqm 1Q44Iafx85llSabeXDEcOdxXWocXCUP5y+CfG+SLeH3LQKK2DCfDxkDMtcFBDaOqJv 3HMHpRbUUmlsMii5otr4W/LYqrG4eJChKhKMXRP42VluhoUFMzhKQPjXcnsbxGFGAB eSZq2hnozQ8Ha+TQTKHYQZsH+7U8XaqMquyRiYdYscnEhoNd02LZ7qoYGMlm0AwCPg wXpHkFmSzOfPQ== X-Mailer: emacs 31.1 (via feedmail 11-beta-1 I) From: Aneesh Kumar K.V To: Catalin Marinas 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 In-Reply-To: References: <20260921144847.501151-1-aneesh.kumar@kernel.org> <20260921144847.501151-3-aneesh.kumar@kernel.org> Date: Wed, 23 Sep 2026 11:23:27 +0530 Message-ID: MIME-Version: 1.0 Content-Type: text/plain X-Rspamd-Queue-Id: 9F51F180002 X-Rspam-User: X-Rspamd-Server: rspam07 X-Stat-Signature: bpcux5rk6efypeqyzs4ew48hh7jaf9kn X-HE-Tag: 1790142819-914408 X-HE-Meta: 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 JtfojP85 U4jabJWvStoW1G4w5dEECG88AKH3EuiWL8/ETrzES2sLh2PkMCVRzZ0EKVNJIx07k7PSRyu6c6TSoLPrrmyhrtdYDRGLvnwtWuRwZEQivTMhcnOSD/fV3SwceIeIw10GQZF6r+Pn+1X8kGw8se+8CpWywvcZubrYADhWd6TBJF8cVyB/iwuXXoApAg7XFDNUo6CHkRHJ76wWUOV6iGCJ1ZUnKgpy2o1CBTvPT5ugGlZmYWrSH339veXRLegvY1vyjNEsEcy1oIaqWLFE= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. Also, if zeroing is added only to the CCA implementation, the allocator must retain __GFP_ZERO for platforms such as pKVM. This would cause the memory to be zeroed twice on CCA. How about extending cc_make_shared() with a flag indicating that the memory must be zeroed, and passing that requirement down to the architecture-specific implementation? The implementation could then zero the memory at the appropriate point: before sharing for pKVM and after the destructive transition for CCA. The allocator could derive this flag from __GFP_ZERO, remove __GFP_ZERO before calling alloc_pages(), and let the sharing operation perform the requested zeroing with the correct ordering. -aneesh