From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 2EB48CA5FA2 for ; Mon, 28 Sep 2026 11:59:59 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 143126B0088; Mon, 28 Sep 2026 07:59:58 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 0CDAB6B008A; Mon, 28 Sep 2026 07:59:58 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id EAF8E6B008C; Mon, 28 Sep 2026 07:59:57 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id BD0D66B0088 for ; Mon, 28 Sep 2026 07:59:57 -0400 (EDT) Received: from smtpin08.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay04.hostedemail.com (Postfix) with ESMTP id 4687C1A01A1 for ; Mon, 28 Sep 2026 11:59:57 +0000 (UTC) X-FDA: 85263027234.08.2174FF5 Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf01.hostedemail.com (Postfix) with ESMTP id 8FD524000C for ; Mon, 28 Sep 2026 11:59:55 +0000 (UTC) Authentication-Results: imf01.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=dF+dytpe; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf01.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1790596795; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=4ubrCI8w3ULvr+Wy09kxqTTxIzrAZwKTgk0GvfF5I+Q=; b=u1YQFpoFK7JfpCOOmi+0OsgnhJunCdk1eXruZG7KoCYRaep/wO0qa2QBBxp/frnrzJdPQ8 hrhBd1vGwzVve+6JC3rw0a3DYpUDO8wbuhdRZRUt94TR9sfSu61IlGEmqP1y0xoWG+FXS/ z2ajiXlwh7Dg6/CBAvbk3jg455N4Y8Q= ARC-Authentication-Results: i=1; imf01.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=dF+dytpe; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf01.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1790596795; b=I8OI7MCTAYKvhzsr50B/wxNKQOgNQLtthD3OcWte+BDRUWbRfWSTpoawCjY9M1al4XeL6Y 9TAXMIXFLzhBaLTcn4s7KORkSYpKxlwXxfBOcFlqlTP6c2+KAmrHXz+HXt1DxNtvzfWx+5 9LrKFrQrl/Vt/3VwO+vFiqYABDokCXE= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id E4159600D1; Mon, 28 Sep 2026 11:59:54 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 90A5A1F000FF; Mon, 28 Sep 2026 11:59:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790596794; bh=4ubrCI8w3ULvr+Wy09kxqTTxIzrAZwKTgk0GvfF5I+Q=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=dF+dytpedS8KF5VBOfaLwmweRWPyyB2yUP/FU4ARxQTSZ4wof0/uc3jPh/GWMyzRP +ygCwX5AHG6drGme6nPQ+Fh6dc7Sv120IAEYYBoQ38X/FVNR5ik5gPBvXQiJtLfUmv iIC784eQhwnZNYNoO6RFtg6HiwaHONlXgjWQYf5rSk0+DVMnxb4N2LFORK8C3KYGM+ 07PhIU4G+EwdqFZFehWixioe4APhhqEQAjfRZEu9sPx2mpYger/suHTL+UNVb5thRI 1LhCH6YrFiDV8i6uGitd/mxWUaPJwbqw9f+pVrhJ33SdqNto7D8NVsDa00mzQHD7Ke Q6c+corn9bKuA== Message-ID: Date: Mon, 28 Sep 2026 13:59:47 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] drivers/char/mem: mmap readonly MAP_SHARED-/dev/zero correctly To: "Lorenzo Stoakes (ARM)" Cc: Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Arnd Bergmann , Greg Kroah-Hartman , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Lance Yang , syzbot+c181d3198e98f8aef8b9@syzkaller.appspotmail.com References: <20260924-fix-dev-zero-readonly-shared-v1-1-153c2111e323@kernel.org> <9efc7bc1-8d9d-49ac-b856-003e81558411@kernel.org> <20260924195826.70a87d6fb78d754bc73892cd@linux-foundation.org> <95e91b4a-ca16-49a3-9a1a-5b38afa6f08d@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: s7uqhjtrtagejc76ahgyqrcny5j81r16 X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 8FD524000C X-HE-Tag: 1790596795-497061 X-HE-Meta: U2FsdGVkX18fkc1g65QnHNiBB2vHnTUkOTqYG9nS1MQbHM2smmtAZKnZa0qg5/aew7wXKvjtK0Nd7lmK7r25znf76LxAkz2PALvDhPTzLe0unBAyF3a0rt3nAnImZin4YTXFZkeGe23nh5CTyVR05GhtezDlq+PQwP//2SwyjFDWKl0XT7J8YMlNWa+LdmVSDJC0Yz8bFOHPCP2iS45Qb4tchLOqvlWstnBvYvAEnOsS6O1vwMn5gIVi2S52VOAFSCOBANdarVvXEMO4vl5gsuTIksChHmDFmDvRZgtxnpZ3hBSWshdhL5C+PnXjZE56MfM/XPfH1bHx/zBFmZQqjvBBSwluy9lS0EuVcvNopkPF7zpGn2Y2bz/C5s8NuRCDtwQiyVLnp1bPmpGHKgpNQkfnQy+G18+HIvMcX4XOfzta7fwCttojHfT0xzw4dS/ptChub43ZTB4HlzVvZvTx88lZehbNPP42ygP8vRpFz3FpGgOU4mtvlcoNtXNiPJOame5P6bZVzfbkFIlA85F4/lRzSaMzvEp/SpEsxYGKmgGELtgUAq3gUz6chJQJo9+n/aPzg4HcaYXE/tofZf4QOi54kLJGIi00S1yh4CZ1jobZuKcipMpeAeWFA+8Vn6DjaRU74S16ekf2kAQ6Clhdv+mWIw0m4bmOmBonOZHKtGuycnQi67Cpu51HxHj71M0SRUx3p0TKsIegdA+t+sTEeMIScBrwiCynZzo+kySRiTnPMZ+RPAqwLvCnLwiBAEzcOQBtSjIIxH7gSv6txmuK2IQdgiuxERn3xLGpvGT0TFz3P3l7CBgErFeQigvzkml6YaHufXVMWcxRCGU56Wh0zXemPrGCiUherv70922y8LgC2qByrclrk04V1cxgY2dTtVI2BT952CvoPwgdFlHirf104Uk/225hz/qGhgtC11qREKExacAKcu5A66QM7GlAIP8O2xvveFjIucMNJMa B17paCux t2DkVIVLXqhC/rEdy38VH4GdyAvWuzRgGUedElXSXW4U5JlxC5DN/E4nFwLhYzLEnK1AiYVNC62EX3YbGvntfQ1wVXJr4zHzsQEvBRSmy/9dH7BUmoklO7h/oiExEFnGT95wCYoImeFEe3O4oSbudYoIXdR5BQ+vWnBtM7+T96/b54Ofvjs9MMo10s6x+hEi+YwdlUXiprBAGsaXY/HKshQAsTYZS3FjjEAf8mkJKYVj/pybCeGTmgxjfXnd1PKrJB9NG42uRGxd+4UmOplqPdAAhdILeeopQEEoMfPdc9EJw5EmHSHQyWZyvKwHoQez98en4gTMknKBQ1LbEJ1m2PFdb3I3c4asZ6+2pCsZYOMDB4HHxk8DGULtC9gew/h+Y8JKblyIZV/UZU4gT9xlIyAYHNy6tI0ty5H+fS0nJ+coFcQODEuQNVuF2xAVoNUBsuVnomHaYm//77nYDHPEuVdn7pA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: > > It then becomes a question about how to get readonly memory. > >> >> We can >> >> a) Disallow allocating an anon_vma and fail gracefully. So only a shared >> zeropage could ever get mapped there. Might break the s390x >> mm_forbids_zeropage(). But given that's only used in hypervisors like QEMU, >> unlikely. > > Something like: > > /* about to maybe prep anon_vma */ > > if (!vma_cow_mapping(vma) && vma_test(vma, VMA_MAYSHARE_BIT)) { > /* don't prep anon give zero page */ > } > > ? Something like that, but really just failing directly from anon-vma preparation in a !vma_cow_mapping(vma), maybe using a special error code. But the less we have to hack around this special case, the better. > > I think though it's surely the only case (I hope!) where you can possibly be > both anon (as in missing vm_ops) and !CoW? I hope? :) > > So it feels better to fix it at the source. > > OTOH maybe it's worth special-casing so we don't allocate on read. > > But that brings me to c)... > > >> >> b) Do what Lorenzo proposes. This will allocate real memory. Someone decided to >> use MAP_SHARED, for unknown reasons, so I'd assume it's unlikely that >> something breaks, but you have a point. > > I would say this patch is the right fix for the moment to fix the assert, and we > can chase up with other approaches afterwards. It will get backported, so we better be careful. IIUC, the real change is only for /dev/zero users that fd = open("/dev/zero", O_RDONLY); mmap(fd, MAP_SHARED, PROT_READ); Which is rather something odd to do indeed. Letting AI do some digging ... it says there are not known users (I don't trust it, but I suspect at least there are no prominent users, which makes sense ...). Getting shared memory when requesting MAP_SHARED does sound like the right thing to do ... So let's try this: Acked-by: David Hildenbrand (Arm) -- Cheers, David