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imf22.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=GrdIEGT5; spf=pass (imf22.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1777040377; a=rsa-sha256; cv=none; b=qCwodyJdS+FKUWSJ1HdvKpZH4b/SFVjQ+pusn46YoZWTelyEJLmKT5iWgQOnfC1N21Sz5z KaQAcuMY2lnFAKiLscME/lEyygKcVNpmzN2/9BZYs12GRASsEZ9kR0Gl6USH5YnuNCN16s JCGhF6Oja0qvrBftwkawkzdO4miy3DA= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by tor.source.kernel.org (Postfix) with ESMTP id 5F52460128; Fri, 24 Apr 2026 14:19:36 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id D526FC2BCB2; Fri, 24 Apr 2026 14:19:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777040376; bh=CxHM58NpmQrbLpCcUPXfUThwYQpzZAqUhNDe2CK5d54=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=GrdIEGT5dd29aeRSp9p9Xj/eqqtbMHDNGU3SqHHSNemLYtVxmK/iokYP3Xk+RTCcC OyrtHWW9npnZScQ958cViCG6tx7M/zz/bj5LwaXeUzhs5rMmAUC6fKinkMLeZbxbv4 zs19Cpf6pvfYqFQQ79DJyG0mP1YE5VpAPtB/DSb5CTWQ05zVVyJvUoCp3C+L5zYyN2 PoMfFRXHPJWFBjRMVEPacWU1d3vaB2MxyQY1Pv2jdbW+90z7A/2nwWt/764WOA7MIN 9x1MZCjjyx8JxlWTh3K49J7IUP9EcHx0eJALHygs/tizIuycfm9LB3KUbXCm9fMH7z qmJIyTgr/IQqw== Message-ID: Date: Fri, 24 Apr 2026 16:19:32 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/gup: honour FOLL_PIN in NOMMU __get_user_pages_locked() To: Andrew Morton , Greg Kroah-Hartman Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jason Gunthorpe , John Hubbard , Peter Xu References: <2026042303-vendor-outright-b9d2@gregkh> <20260424063817.552a894d4b5bfa4de40792fa@linux-foundation.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: <20260424063817.552a894d4b5bfa4de40792fa@linux-foundation.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: qd8hgaemuc8kcp7xun5t9rfapb37zsnm X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: F00C3C000A X-HE-Tag: 1777040376-105434 X-HE-Meta: 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 axGSx0Ak dvDqSdQrlUeSj9j0mleTZiojZ3Ag+lj//L0VtKSt9A0vQ3871Lf7lxTvdQdNUKacYkCgvtPc9/4lxfMbJvg5re5PqLGh4x9rithvpfNyRHZFsLDKGgQgt/p+35LvJmtxCtOVfxtnLoNzxjxu/jxII8d0q93e8eTStYTvSwhUBnCIdiqZYHnOSjq30CDt1sfSQ8MbvOTeBErc1G5danXd536ZgGHGpyJhYFmitHUT9cuyWVYfO4t7/sK64BBcvYYeQteIILanx0/vDPB2c7YrTouadmFpXlECNkz67iPdcIqnihxoRyIirP5wJ6pBb2cn34pUmqnuKUiR/HoFQtboKsmuLn2K8O1HfwmDNnM0Hm7+WWjRpOBLJtQ34316DHLv2ZeD6xQ8eaEwaGKgEM+m8cBDJ9X3aADRPAdm94X5QTPG8DLA= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/24/26 15:38, Andrew Morton wrote: > On Thu, 23 Apr 2026 16:28:04 +0200 Greg Kroah-Hartman wrote: > >> The !CONFIG_MMU implementation of __get_user_pages_locked() takes a bare >> get_page() reference for each page regardless of foll_flags: >> if (pages[i]) >> get_page(pages[i]); >> >> This is reached from pin_user_pages*() with FOLL_PIN set. >> unpin_user_page() is shared between MMU and NOMMU configurations and >> unconditionally calls gup_put_folio(..., FOLL_PIN), which subtracts >> GUP_PIN_COUNTING_BIAS (1024) from the folio refcount. >> >> This means that pin adds 1, and then unpin will subtract 1024. >> >> If a user maps a page (refcount 1), registers it 1023 times as an >> io_uring fixed buffer (1023 pin_user_pages calls -> refcount 1024), then >> unregisters: the first unpin_user_page subtracts 1024, refcount hits 0, >> the page is freed and returned to the buddy allocator. The remaining >> 1022 unpins write into whatever was reallocated, and the user's VMA >> still maps the freed page (NOMMU has no MMU to invalidate it). >> Reallocating the page for an io_uring pbuf_ring then lets userspace >> corrupt the new owner's data through the stale mapping. >> >> Use try_grab_folio() which adds GUP_PIN_COUNTING_BIAS for FOLL_PIN and 1 >> for FOLL_GET, mirroring the CONFIG_MMU path so pin and unpin are >> symmetric. > > Battle of the bots? > https://sashiko.dev/#/patchset/2026042303-vendor-outright-b9d2@gregkh It references the if (pages && !(flags & FOLL_PIN)) flags |= FOLL_GET; I'm not sure if there is actual NOMMU code that triggers it. For example, uprobes uses that pattern, but I suspect that that's not a thing on NOMMU. Probably best to just squash: diff --git a/mm/gup.c b/mm/gup.c index ad9ded39609c..44bd28cf6e00 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -1988,6 +1988,9 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start, if (!nr_pages) return 0; + if (pages && !(foll_flags & FOLL_PIN)) + foll_flags |= FOLL_GET; + /* * The internal caller expects GUP to manage the lock internally and the * lock must be released when this returns. -- Cheers, David