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[80.187.80.144]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-8013cdf14ddsm97613996d6.25.2025.09.30.08.32.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 30 Sep 2025 08:32:30 -0700 (PDT) Message-ID: <85f852f9-8577-4230-adc7-c52e7f479454@redhat.com> Date: Tue, 30 Sep 2025 17:32:25 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/ksm: fix flag-dropping behavior in ksm_madvise To: Jakub Acs , linux-mm@kvack.org Cc: Andrew Morton , Xu Xin , Chengming Zhou , Peter Xu , Axel Rasmussen , linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20250930130023.60106-1-acsjakub@amazon.de> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+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: <20250930130023.60106-1-acsjakub@amazon.de> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 60NOWmqlVWfFq2F_lQBc6cH2R9SGZ3YtQlMvm9A4iks_1759246352 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; 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Specifically, a VMA which has a valid pointer > to vma->vm_userfaultfd_ctx, but no UFFD flags in vma->vm_flags. > > The inconsistency is caused in ksm_madvise(): when user calls madvise() > with MADV_UNMEARGEABLE on a VMA that is registered for UFFD in MINOR > mode, it accidentally clears all flags stored in the upper 32 bits of > vma->vm_flags. > > Assuming x86_64 kernel build, unsigned long is 64-bit and unsigned int > and int are 32-bit wide. This setup causes the following mishap during > the &= ~VM_MERGEABLE assignment. > > VM_MERGEABLE is a 32-bit constant of type unsigned int, 0x8000'0000. > After ~ is applied, it becomes 0x7fff'ffff unsigned int, which is then > promoted to unsigned long before the & operation. This promotion fills > upper 32 bits with leading 0s, as we're doing unsigned conversion (and > even for a signed conversion, this wouldn't help as the leading bit is > 0). & operation thus ends up AND-ing vm_flags with 0x0000'0000'7fff'ffff > instead of intended 0xffff'ffff'7fff'ffff and hence accidentally clears > the upper 32-bits of its value. > > Fix it by changing `VM_MERGEABLE` constant to unsigned long. Modify all > other VM_* flags constants for consistency. > > Note: other VM_* flags are not affected: > This only happens to the VM_MERGEABLE flag, as the other VM_* flags are > all constants of type int and after ~ operation, they end up with > leading 1 and are thus converted to unsigned long with leading 1s. > > Note 2: > After commit 31defc3b01d9 ("userfaultfd: remove (VM_)BUG_ON()s"), this is > no longer a kernel BUG, but a WARNING at the same place: > > [ 45.595973] WARNING: CPU: 1 PID: 2474 at mm/userfaultfd.c:2067 > > but the root-cause (flag-drop) remains the same. > > Fixes: 7677f7fd8be76 ("userfaultfd: add minor fault registration mode") > Signed-off-by: Jakub Acs > Cc: Andrew Morton > Cc: David Hildenbrand > Cc: Xu Xin > Cc: Chengming Zhou > Cc: Peter Xu > Cc: Axel Rasmussen > Cc: linux-mm@kvack.org > Cc: linux-kernel@vger.kernel.org > Cc: stable@vger.kernel.org > --- If we want a smaller patch for easier backporting, we could split off the VM_MERGEABLE change into a separate patch and do all the other ones for consistency in another Reading what we do VM_HIGH_ARCH_BIT_* , we use BIT(), which does #define BIT(nr) (UL(1) << (nr)) So likely we should just clean it all up an use e.g., #define VM_NONE 0 #define VM_READ BIT(0) #define VM_WRITE BIT(1) etc. So likely it's best to do in a first fix #define VM_MERGEABLE BIT(31) And in a follow-up cleanup patch convert all the other ones. Sorry for not thinking about BIT() earlier -- Cheers David / dhildenb