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Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jeff Xu References: <4f66d89a-631a-43eb-b4f9-c9a0b44caaae@redhat.com> <692f9624-e440-4cf2-8202-861c679ddb73@lucifer.local> <0ad414a1-9a57-4050-a7b6-fdea08aebbd1@redhat.com> <507a09cf-291c-4886-92e7-9d9cc294a247@lucifer.local> 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 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: <507a09cf-291c-4886-92e7-9d9cc294a247@lucifer.local> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: NXdLRtVJ9TjjwuqWs8vdVMxCxewy2DOaeig_bKHHzaA_1753436776 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 848B840003 X-Stat-Signature: jyz5xedcr8imgmn3stiux39nttaaxs3x X-Rspam-User: X-Rspamd-Server: rspam11 X-HE-Tag: 1753436779-546071 X-HE-Meta: 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 a9cBo9ex OtR3fg/O8PeP9JIgglxDZSEBQUv+3tulZC0J1412I16h5htlckcuiLYE7qEj0rLivXBMIHAWIrLc+w+UsMiL5rqOCq0o2CExsAzcfmJm7ibOVX53o5X+kQRw+6TwpsRocqzhSvZE6hSSFu2C/aIsRUHiposaCcF+Ds5hCAPPHIrtiy5RVFY/cQ9vuB83bl3XeKM0Dxkmd36bPu2K1l/bbJdyZZyicJNtJWQigglg4iK3Q7C/vIah0Czo0s0NzqEqCpS+qy2fSKmNrZwJ3jKUqOpDDKcUldJCmXPUnM11FxGr2i7h4/PDZHG9Mk9myTLpkWQrKgWRvTCrCeSRyoCI15tRwa88+TQggWTOz25Pd4Hs/IEYxAnpqQUOZPfM3WfL3bVqnrcNsIHqEbWagoOeLcbn/kzM5VqTZ6W5fC114yv7yUVb9SZDU/LoST3j1RcYhmdGcQxuEJq+xxOl2D5vY8i1xdQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >> >> Well, there is long-term pinning that can break COW and other weird stuff >> like FOLL_FORCE. Most of the latter probably holds the mmap lock in write >> mode. Probably. > > Well GUP uses read lock. Right, so it can race with MADV_DONTNEED. > > FOLL_FORCE won't override anything as we have this check in check_vma_flags(): > > if (write) { > if (!vma_anon && > !writable_file_mapping_allowed(vma, gup_flags)) > return -EFAULT; > > if (!(vm_flags & VM_WRITE) || (vm_flags & VM_SHADOW_STACK)) { > if (!(gup_flags & FOLL_FORCE)) > return -EFAULT; > /* > * We used to let the write,force case do COW in a > * VM_MAYWRITE VM_SHARED !VM_WRITE vma, so ptrace could > * set a breakpoint in a read-only mapping of an > * executable, without corrupting the file (yet only > * when that file had been opened for writing!). > * Anon pages in shared mappings are surprising: now > * just reject it. > */ > if (!is_cow_mapping(vm_flags)) > return -EFAULT; > } > } > > With: > > static inline bool is_cow_mapping(vm_flags_t flags) > { > return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE; > } > Not sure what you mean. Using FOLL_FORCE you can write into MAP_PRIVATE R/O mappings. Particular useful for installing breakpoints into loaded executables etc. is_cow_mapping() tells you exactly that: the only place where we can have anon folios is when we have a MAP_PRIVATE mapping (!VM_SHARED) that can be writable, for example, through mprotect(PROT_WRITE) (VM_MAYWRITE). A MAP_PRIVATE R/O file mapping matches is_cow_mapping(). > So - we explicitly disallow FOLL_FORCE write override for CoW file-backed > mappings. > > Obviously if FOLL_FORCE is not set, then we're ALSO not allowed to get past a > FOLL_WRITE and !VM_WRITE situation. > >> >>> >>> Hmm maybe I'll soften on this anon_vma idea then. Maybe it is a 'cheap fix' to >>> rule out the _usual_ cases. >> >> Yeah, something to evaluate. > > I'm thinking more and more we're probably actually safe with !vma->anon_vma || > !(vma->vm_flags & VM_MAYWRITE). I think there are possible races, the question is how much you care about them. In case of CoW-unsharing, you're not actually discarding data, because the page in the anon folio is to maintain a copy of the pagecache page (of course, they can go out of sync, but that's a different discussion). -- Cheers, David / dhildenb