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[2003:d8:2f44:3700:be07:9a67:67f7:24e6]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3b79c4a2f03sm2278296f8f.72.2025.07.31.05.56.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 31 Jul 2025 05:56:26 -0700 (PDT) Message-ID: <214e78a0-7774-4b1e-8d85-9a66d2384744@redhat.com> Date: Thu, 31 Jul 2025 14:56:25 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/userfaultfd: fix missing PTE unmap for non-migration entries To: Sasha Levin Cc: Andrew Morton , peterx@redhat.com, aarcange@redhat.com, surenb@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20250630031958.1225651-1-sashal@kernel.org> <20250630175746.e52af129fd2d88deecc25169@linux-foundation.org> From: David Hildenbrand Content-Language: en-US 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 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAmgsLPQFCRvGjuMACgkQTd4Q 9wD/g1o0bxAAqYC7gTyGj5rZwvy1VesF6YoQncH0yI79lvXUYOX+Nngko4v4dTlOQvrd/vhb 02e9FtpA1CxgwdgIPFKIuXvdSyXAp0xXuIuRPQYbgNriQFkaBlHe9mSf8O09J3SCVa/5ezKM OLW/OONSV/Fr2VI1wxAYj3/Rb+U6rpzqIQ3Uh/5Rjmla6pTl7Z9/o1zKlVOX1SxVGSrlXhqt kwdbjdj/csSzoAbUF/duDuhyEl11/xStm/lBMzVuf3ZhV5SSgLAflLBo4l6mR5RolpPv5wad GpYS/hm7HsmEA0PBAPNb5DvZQ7vNaX23FlgylSXyv72UVsObHsu6pT4sfoxvJ5nJxvzGi69U s1uryvlAfS6E+D5ULrV35taTwSpcBAh0/RqRbV0mTc57vvAoXofBDcs3Z30IReFS34QSpjvl Hxbe7itHGuuhEVM1qmq2U72ezOQ7MzADbwCtn+yGeISQqeFn9QMAZVAkXsc9Wp0SW/WQKb76 FkSRalBZcc2vXM0VqhFVzTb6iNqYXqVKyuPKwhBunhTt6XnIfhpRgqveCPNIasSX05VQR6/a OBHZX3seTikp7A1z9iZIsdtJxB88dGkpeMj6qJ5RLzUsPUVPodEcz1B5aTEbYK6428H8MeLq NFPwmknOlDzQNC6RND8Ez7YEhzqvw7263MojcmmPcLelYbfOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCaCwtJQUJG8aPFAAKCRBN3hD3AP+DWlDnD/4k2TW+HyOOOePVm23F5HOhNNd7nNv3 Vq2cLcW1DteHUdxMO0X+zqrKDHI5hgnE/E2QH9jyV8mB8l/ndElobciaJcbl1cM43vVzPIWn 01vW62oxUNtEvzLLxGLPTrnMxWdZgxr7ACCWKUnMGE2E8eca0cT2pnIJoQRz242xqe/nYxBB /BAK+dsxHIfcQzl88G83oaO7vb7s/cWMYRKOg+WIgp0MJ8DO2IU5JmUtyJB+V3YzzM4cMic3 bNn8nHjTWw/9+QQ5vg3TXHZ5XMu9mtfw2La3bHJ6AybL0DvEkdGxk6YHqJVEukciLMWDWqQQ RtbBhqcprgUxipNvdn9KwNpGciM+hNtM9kf9gt0fjv79l/FiSw6KbCPX9b636GzgNy0Ev2UV m00EtcpRXXMlEpbP4V947ufWVK2Mz7RFUfU4+ETDd1scMQDHzrXItryHLZWhopPI4Z+ps0rB CQHfSpl+wG4XbJJu1D8/Ww3FsO42TMFrNr2/cmqwuUZ0a0uxrpkNYrsGjkEu7a+9MheyTzcm vyU2knz5/stkTN2LKz5REqOe24oRnypjpAfaoxRYXs+F8wml519InWlwCra49IUSxD1hXPxO WBe5lqcozu9LpNDH/brVSzHCSb7vjNGvvSVESDuoiHK8gNlf0v+epy5WYd7CGAgODPvDShGN g3eXuA== Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 31.07.25 14:37, Sasha Levin wrote: > On Tue, Jul 08, 2025 at 05:42:16PM +0200, David Hildenbrand wrote: >> On 08.07.25 17:33, Sasha Levin wrote: >>> On Tue, Jul 08, 2025 at 05:10:44PM +0200, David Hildenbrand wrote: >>>> On 01.07.25 02:57, Andrew Morton wrote: >>>>> On Sun, 29 Jun 2025 23:19:58 -0400 Sasha Levin wrote: >>>>> >>>>>> When handling non-swap entries in move_pages_pte(), the error handling >>>>>> for entries that are NOT migration entries fails to unmap the page table >>>>>> entries before jumping to the error handling label. >>>>>> >>>>>> This results in a kmap/kunmap imbalance which on CONFIG_HIGHPTE systems >>>>>> triggers a WARNING in kunmap_local_indexed() because the kmap stack is >>>>>> corrupted. >>>>>> >>>>>> Example call trace on ARM32 (CONFIG_HIGHPTE enabled): >>>>>> WARNING: CPU: 1 PID: 633 at mm/highmem.c:622 kunmap_local_indexed+0x178/0x17c >>>>>> Call trace: >>>>>> kunmap_local_indexed from move_pages+0x964/0x19f4 >>>>>> move_pages from userfaultfd_ioctl+0x129c/0x2144 >>>>>> userfaultfd_ioctl from sys_ioctl+0x558/0xd24 >>>>>> >>>>>> The issue was introduced with the UFFDIO_MOVE feature but became more >>>>>> frequent with the addition of guard pages (commit 7c53dfbdb024 ("mm: add >>>>>> PTE_MARKER_GUARD PTE marker")) which made the non-migration entry code >>>>>> path more commonly executed during userfaultfd operations. >>>>>> >>>>>> Fix this by ensuring PTEs are properly unmapped in all non-swap entry >>>>>> paths before jumping to the error handling label, not just for migration >>>>>> entries. >>>>> >>>>> I don't get it. >>>>> >>>>>> --- a/mm/userfaultfd.c >>>>>> +++ b/mm/userfaultfd.c >>>>>> @@ -1384,14 +1384,15 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, >>>>>> entry = pte_to_swp_entry(orig_src_pte); >>>>>> if (non_swap_entry(entry)) { >>>>>> + pte_unmap(src_pte); >>>>>> + pte_unmap(dst_pte); >>>>>> + src_pte = dst_pte = NULL; >>>>>> if (is_migration_entry(entry)) { >>>>>> - pte_unmap(src_pte); >>>>>> - pte_unmap(dst_pte); >>>>>> - src_pte = dst_pte = NULL; >>>>>> migration_entry_wait(mm, src_pmd, src_addr); >>>>>> err = -EAGAIN; >>>>>> - } else >>>>>> + } else { >>>>>> err = -EFAULT; >>>>>> + } >>>>>> goto out; >>>>> >>>>> where we have >>>>> >>>>> out: >>>>> ... >>>>> if (dst_pte) >>>>> pte_unmap(dst_pte); >>>>> if (src_pte) >>>>> pte_unmap(src_pte); >>>> >>>> AI slop? >>> >>> Nah, this one is sadly all me :( >> >> Haha, sorry :P > > So as I was getting nowhere with this, I asked AI to help me :) > > If you're not interested in reading LLM generated code, feel free to > stop reading now... > > After it went over the logs, and a few prompts to point it the right > way, it ended up generating a patch (below) that made sense, and fixed > the warning that LKFT was being able to trigger. > > If anyone who's more familiar with the code than me (and the AI) agrees > with the patch and ways to throw their Reviewed-by, I'll send out the > patch. Seems to check out for me. In particular, out pte_unmap() everywhere else in that function (and mremap.c:move_ptes) are ordered properly. Even if it would not fix the issue, it would be a cleanup :) Acked-by: David Hildenbrand -- Cheers, David / dhildenb