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b=oKGrAF3Y6UveLrA2UOY5wx0zRSLKy04JzXsiOxj3ysMaZPU993WGae1/63tiWDXuP StuarUNmsSOtX8r+bhrwT7f76+/Ej4S0PuDqW48GA2hdZiKejogy7nZLg4/rIAkM1V 1mde+oIHX9VOk5yzlXz8e7Rjqv8uD5VER42XZoR5D5bJ6r+RvcrM1ZS01NXWIE75Ox x4xyDy5KgbHYLxmsY6c3XTK1IFNOCxux1W7wISTv+yyuwofen4r+m/Cl84l102VPW5 RxvtZNU8JJR/pQEXXTPzJAZ85feEKmEQokKl3zjvM+7ePNjY+AbnDav77u3sKEo8AL UcRX5TS64TU5w== Message-ID: Date: Mon, 5 Oct 2026 12:27:50 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/gup: fix NULL pointer dereference in fixup_user_fault() To: Lance Yang , akpm@linux-foundation.org Cc: neganhat@gmail.com, jgg@ziepe.ca, jhubbard@nvidia.com, peterx@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20261004125601.c06953a6f0660df9859b0c2a@linux-foundation.org> <20261005021957.5254-1-lance.yang@linux.dev> 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: <20261005021957.5254-1-lance.yang@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 3C1D9140005 X-Stat-Signature: 5jq7ruy8eqddy43g374ah58bousuhgr9 X-Rspam-User: X-HE-Tag: 1791196078-818123 X-HE-Meta: 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 LFb037Ys iqCzWOtMQ49mh/TVSFLIwCx51ZIQOSYLKdefP04lLLQYXSQBtc1ekRSMowKL5+qI7qCwfK47gkzvk99c06ClYvA1dt+nhDVDZ4W56OFoLa9DNDaHLnfvGt4Z8IatqNSvBZ/B43+JnyJPEzAsWEyvKN3PFV0HAm3wvdSkgIJiH8s+LKZTRLlpYVCE8PaiXtbU2/9GZdosUMLQeezAcbpKLmBaiyQ9odoBJUBGLRWAL00KIJiIpBKZT9p5+Dc9OSrB3W8bKW8G8CZkaMOqCMwob39n345e/5J4mSePuFvi6KuCwhr28Sq3QKW86R9aauEu6bUYNKz5WniQ0opY6l6lCLj2WPP7jonpdmCI5 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 10/5/26 04:19, Lance Yang wrote: > > On Sun, Oct 04, 2026 at 12:56:01PM -0700, Andrew Morton wrote: >> On Sun, 4 Oct 2026 02:48:46 +0700 Nguyen Duy Nhat Anh wrote: >> >>> In fixup_user_fault(), the 'unlocked' parameter is checked for NULL >>> early on, allowing callers to pass NULL if they do not track whether the >>> mmap lock was dropped. >>> >>> However, if handle_mm_fault() returns VM_FAULT_COMPLETED, line 1597 >>> dereferences 'unlocked' directly (*unlocked = true) without checking >>> if it is NULL. Callers like s390's pci_mmio.c pass NULL for 'unlocked', >>> leading to a kernel NULL pointer dereference when VM_FAULT_COMPLETED >>> occurs. >>> >>> Fix this by checking if 'unlocked' is non-NULL before assigning to it. >> >> This code is too subtle so you aren't the first to attempt to "fix" it. >> >> The key hint is in the kerneldoc: >> >> * @unlocked: did we unlock the mmap_lock while retrying, maybe NULL if caller >> * does not allow retry. If NULL, the caller must guarantee >> * that fault_flags does not contain FAULT_FLAG_ALLOW_RETRY. >> >> Trace through the >> FAULT_FLAG_ALLOW_RETRY/VM_FAULT_COMPLETED/VM_FAULT_RETRY logic >> to confirm that the null deref is a cant-happen. > > IIUC, that's indeed a can't-happen :) https://lore.kernel.org/linux-mm/c44e5924-091f-43ed-8cff-34e87c3d9763@kernel.org/ People should stop trusting tool output and use their brain ;) If you did mmap_read_lock() and wouldn't have a way to indicate that to the user something would be seriously messed up. -- Cheers, David