From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0E39D2D3733 for ; Mon, 28 Sep 2026 11:59:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790596796; cv=none; b=qqiYYzghonnHV88L0x1v8RYEox2YcdNJiVRiHwzWMHIpKoOqcIasrTg9ynFZyZ8fEXFsQi5pJ9msxjY5++y0tgbKE5bGys5ZvwHz84kltgHjA0KuCUrMUJ+GF/oqpmfm7F4lMaYgpYo6ws2p0mjhSjKZI7wynA/b0/3acFF9OD4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790596796; c=relaxed/simple; bh=ONXWuGJSEhtdkBjLPYaEyIyjxeJJHaxGYoxwqBSS2gU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=o3pBpyMzAU2xjq79lbjZS/nxCsz+1c8PkzR8hL0nbaVItvNgIZIVavcmwXtFVFwppSMfF3VIHLgf4FLP+sFhlJowpRCw4nimOMbCwwlR1EM9Cxh42vbY/wyy7KYndoHpVtHzAdK+qxP57FraYSNMbbP7t5NqYgXNB/oAPjKPlQw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=dF+dytpe; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="dF+dytpe" 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 > > 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