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h=From:To:Cc:Subject:In-Reply-To:References:Date; b=gfeKGD7Zx1t68oVKPD3pJdIls6095LdyIYbLzcbi2lWu0uZc4Ag6czwUu/3rMOVvu 5DkKG7QVnUdoUJyu1nzlZlhO5qsE27dTDKv4KVUeUxS/0fGMmnV48ACdIVTacKhMLM Kp2Etb2iursi6SlXAk2q9a0Y6ZYslY12NqaBGyj2gFaxqHvu92+46mMlc5J8MMsSdD HGiK3yV1AdehOFvWHLjpPcT/57yOMwlxfev62t4C+oD/yqe8IWaG9c7HzZhanNfFx0 V7SG8BRuoVDTolq0VsrAo7+8om8W5vw2V1IN/4OrxXJcM2FyWTn+Eqh0SDfkr9xU/H iMGeRqREn7XNA== 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 15:58:25 +0530 Message-ID: MIME-Version: 1.0 Content-Type: text/plain X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: E87F5C0005 X-Stat-Signature: cg73yrupbm65i41htg5f3jkskkdx836d X-HE-Tag: 1790159315-256130 X-HE-Meta: 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 VtnRZCb0 pKSOmZsBhXOO3LdHQW+gR0Vyw5jsm9PRu0PVCFrrlH1EdFSBuAv4P8nFExOMK5kz7UvtCgKjn6sCPRWi+pMzEWKl0VFDqimOUtiRIYmiP730hRLpzeb1jkUMeB13FmGKBZP6C1u+8qolG6qGi/mhT+E7RwWoh0AggOm/HllECyJpJAWBtQuR2phmwrk2bbjW58QAM4YZaqLfrnpzDZs42zVvFgpEQWcJm47Ok/4vdnUfjtn1GzD5wUOYeu47H0ls0FB1t4RUEs9Xsbis3EVT/l/BVow== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. -aneesh