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b=sJ0tb4HGbbhdYuMn0qIVUVj0alcqc8Q8ueyANCykxd2TCkFMuXGBibV6SUCUQZnJ1 x+mM2INukLxebNBZxFbGvdrmiu1rWCripO9nlSufN7hL3SA5Bf6MLLjOuiXtpramAT BzIARPtuvBTYwe4MUV5F9fJaE/ermR4wDGggGMaOZReQCCdZKK2UXeizHWBFozJrhJ 4LZJp2lg0zfYlwCvahJuCmBRGPLtaQcm8sLCPWaamQG+ulNdEkwiWvtnCKjXzUBES0 /aO6NIr1ZOvk31kOtBG3I+GmMH96jRBKuXO52YeLLL77PvWT/XBw+5WsalZHD2py27 0ejZnETyN8Wiw== Message-ID: Date: Fri, 24 Apr 2026 14:16:21 +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 Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Jason Gunthorpe , John Hubbard , Peter Xu References: <2026042303-vendor-outright-b9d2@gregkh> <2026042431-charter-ranging-597c@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: <2026042431-charter-ranging-597c@gregkh> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: uhay7o8x4mex6chekjnmneus9kwsrw38 X-Rspam-User: X-Rspamd-Queue-Id: 4A00B140005 X-Rspamd-Server: rspam05 X-HE-Tag: 1777032985-368608 X-HE-Meta: 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 qjofl9a3 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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 4/24/26 13:31, Greg Kroah-Hartman wrote: > On Thu, Apr 23, 2026 at 05:55:56PM +0200, David Hildenbrand (Arm) wrote: >> 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! :) ) > > True, sorry, I guess people can "assist", I should have added that. > > If Andrew's tools automatically pick this up then: > > Assisted-by: David Hildenbrand I think we usually use Suggested-by:, but really I was just joking :) > >>> 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). > > Ick, I see Andrew already grabbed this so I'll just leave it for now, > thanks for the help and review! Andrew, can you add "While at it, don't return NULL pointers in the page array, as this is really not expected for GUP users; instead, just fail and return -EFAULT."? Thanks! -- Cheers, David