From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793011651; b=3Ge0Mqg4WOg0fAj4dpNjWVC5LG9IkviM01xoC29JQgGSPQREwyj/IRIBNbHgqE1hYfOx THdusUglRI8atEjBYeI5a7JMTgJc3U/cQQ7LR4K9B5xD6h93TGq+qielTkhMR3boHIoPs xUREz/j9tmR3g+wqozgonILgquO8zqIGMK7QVIt29ia6HmAYULE5W/pO9UcPSwZnrJ0B5 Jz4NKzpS6ZgR0Mn6leh2rRRNe2iO2zwxOKJflPZgsV6/yNWXXHfLaDb+zZlZrfciaD2BW EFJbA0POYK5yynCQfNCpu5TI0EcTvcq8krUZIwYWA3265IAIpNg3EYZT7CsB2bHsX7Q== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793011651; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:From:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=PCm/g3rUcyKZ1+NtoTVxUUIZZdtpcxA7CoFy2Hyn1ag=; b=rYB0WMDHC3jnxJglz2YM3JDjNG3M4JDVD2AWqGK4RCcfMi7Nb+uBmR0PxY35xopF0qbs BKCwwuhbzKDTol8FNDLAb/Oy7F6blJWoWLuhhCmxovBa+gkvhrFRbUsfDFkDlO2PRsu9v hr/n10zis4By88L7+A3Bacmj+wTdgmKo3MkBJm4ZUEXLsZSfMXTCSEKSxk4o/TucVrHAq W11O7qNzCp/XXOgqAM9ZyDx8N33Xni7LDPqR4eg9oxstgCNTD4NmCN6Doq57FWZQaEyod USvS1DwP2Vg9A4x9lxE2RbYpmLZTBcGJb9zfDPXN21zpZxa1Wm+bn1GIsxwWAwfP99g== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=UetnZjY6; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 17CD8606A31B; Mon, 26 Oct 2026 10:47:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=UetnZjY6 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8B8DF1F01825; Mon, 26 Oct 2026 10:47:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793011650; bh=PCm/g3rUcyKZ1+NtoTVxUUIZZdtpcxA7CoFy2Hyn1ag=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=UetnZjY6MfvWbi60HZpHtL7Tekegir2yz8Hvdoty2oYPiw/rHr+dQukKtCnGa7hOK 5ljeICvZdgY2bxtWFBzEw5EAbtx9lXQQ/A0jQ6FI0aIT9LDsQbN2hdCkpYqmMOT7Y0 wh35e//fpN6nmINe6m7AGEn67xiNPOdJf3PJmGwoXIMOKbuyQCPAaRJjetIiAxl3ew /UF7g6huzUt+iwJcYcQ/v4OpK7NHBd3DbbwiHd6hEpvsIPGFol86+9Me9MU2Nb40A6 jUdEPVg445d5TTdkZaYaO4q2oXetuHENgTei/3+kW2Vj7ymsGnbsOS/WuaLPL2Nq8E Qv2lI0dXf6rxw== Message-ID: <190c8e19-d532-4ba2-b493-3510cc785adc@kernel.org> Date: Mon, 26 Oct 2026 11:47:22 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/4] mm: do not unintentionally overwrite atomic VMA flags To: "Lorenzo Stoakes (ARM)" , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jann Horn , Pedro Falcato , Davidlohr Bueso , SJ Park , Kees Cook , Colin Cross Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20261023-fix-vma-atomic-v1-0-8ff4fae1fc85@kernel.org> <20261023-fix-vma-atomic-v1-1-8ff4fae1fc85@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: <20261023-fix-vma-atomic-v1-1-8ff4fae1fc85@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/23/26 15:47, Lorenzo Stoakes (ARM) wrote: > madvise, mlock and mprotect take a copy of a VMA's flags holding only the > VMA read lock, later writing it back under the write lock (in > madvise_update_vma(), mlock_fixup(), mlock_vma_pages_range() and > mprotect_fixup()). > > This was done on the assumption that VMA flags cannot change with a read > lock held. > > However this changed in commit 2ab7f1bbafc9 ("mm/madvise: allow guard page > install/remove under VMA lock"), which allowed VM_ATOMIC_SET_ALLOWED > flags (currently only VMA_MAYBE_GUARD_BIT) to be written concurrently under > a VMA read lock. > > The actual write itself is safe as this is done under the VMA write lock > at which point no further concurrent writes can occur, however at this > point the stale flags mean that these callers can unintentionally overwrite > atomic flags incorrectly. > > The consequences of this are significant - a missing VMA_MAYBE_GUARD_BIT > could result in page tables not being copied on fork (the vma_needs_copy() > check can return false) or being overwritten on file-backed THP > collapse (the file_backed_vma_is_retractable() can return true). > > Resolve the issue by always taking care to retain existing atomic flags > when overwriting VMA flags, replacing the inconsistently named > vma_flags_reset_once() with vma_overwrite_flags() to do so, and always > invoking it when flags are overwritten like this. > > Reads are a problem too - madvise, mlock and mprotect all compare VMA flags > without a VMA write lock held via vma_flags_same_pair(). > > This has more serious impact, as concurrent changes to atomic VMA flags can > cause two sets of VMA flags to be wrongly considered different. > > For mprotect and madvise this causes a useless split, but mlock is impacted > more seriously as it can result in mm->locked_vm underflow. > > Fix this by updating vma_flags_same_pair() to disregard > VM_ATOMIC_SET_ALLOWED VMAs - VMA equality is the same as VMA mergeability, > and atomic VMA flags are disregarded when comparing VMA flags for merge - > so this is justified semantically. > > Additionally, remove the redundant vma_flags_same_mask() and the unused > vma_flags_same() as part of the change, and replace VM_ATOMIC_SET_ALLOWED > with VMA_ATOMIC_SET_ALLOWED, using the new VMA flag API. > > Finally, update userland VMA tests to reflect the changes. > > Suggested-by: David Hildenbrand (Arm) > Fixes: 2ab7f1bbafc9 ("mm/madvise: allow guard page install/remove under VMA lock") > Cc: > Signed-off-by: Lorenzo Stoakes (ARM) > --- Nothing jumped at me Acked-by: David Hildenbrand (Arm) -- Cheers, David