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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776959761; bh=0L5e/EPnegp4JHS0wXmUj5FxsmR4LIX+3gaFnChJO6s=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=pEhBq3vdwrjld+IR22eeekTh4+a7rsdskikeEVkX/tZ9JK7SIwOSiFuyezbIQGF/G 4+It6V9BojzewleJ5ErtRo+QcDhD5h6h6tNoxhbHJcLOQlyGIiv7GM5ade5pHMGVdp zt8SoXvHT3ZjeQYk0TYI2Wg11xUfCw2GGzreWVwMTw1oRzXKKQx/sYTxDPWQDIrf7D q2L92KkHygasjzOvRtHqekUD7PJZnTw7/0HHo9v8nJxU6eZ+B5A/8wQ6aRNpL5J82v BodwqsZT80AwLa79kq5nr9f/boblucydGsIpyUbpvx88yO92wNK3swcNny/QtOX59R a1LvbZZBpupJg== Message-ID: Date: Thu, 23 Apr 2026 17:55:56 +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: Greg Kroah-Hartman , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Andrew Morton , Jason Gunthorpe , John Hubbard , Peter Xu References: <2026042303-vendor-outright-b9d2@gregkh> 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: <2026042303-vendor-outright-b9d2@gregkh> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam10 X-Stat-Signature: hn85h89ecxq9dsps3kcigzewdofgky3y X-Rspam-User: X-Rspamd-Queue-Id: 3F6BE80006 X-HE-Tag: 1776959762-932606 X-HE-Meta: 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 gdTKhBaB 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/23/26 16:28, 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. > > Cc: Andrew Morton > Cc: David Hildenbrand > Cc: Jason Gunthorpe > Cc: John Hubbard > Cc: Peter Xu > Reported-by: Anthropic > Assisted-by: gkh_clanker_t1000 Assisted-by: David :( (no, I'm not a tool! :) ) > Signed-off-by: Greg Kroah-Hartman > --- > v2: - drop huge comment > - rework error return value based on David's suggestion (heck, > pretty much the full patch was written by him now) > Link to v1: https://lore.kernel.org/r/2026042334-acutely-unadorned-e05c@gregkh > > mm/gup.c | 13 ++++++++++--- > 1 file changed, 10 insertions(+), 3 deletions(-) > > diff --git a/mm/gup.c b/mm/gup.c > index ad9ded39609c..2f6f95a167af 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -1983,6 +1983,7 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start, > struct vm_area_struct *vma; > bool must_unlock = false; > vm_flags_t vm_flags; > + int ret, err = -EFAULT; > long i; > > if (!nr_pages) > @@ -2019,8 +2020,14 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start, > > if (pages) { > pages[i] = virt_to_page((void *)start); > - if (pages[i]) > - get_page(pages[i]); > + if (!pages[i]) > + break; Best to mention that change in the patch description. I really think this is the right thing to do (returning NULL in the page array is just very dubious). Acked-by: David Hildenbrand (Arm) -- Cheers, David