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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org Cc: Peter Xu , linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20250603-fork-tearing-v1-0-a7f64b7cfc96@google.com> <20250603-fork-tearing-v1-1-a7f64b7cfc96@google.com> 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/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz 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+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: <20250603-fork-tearing-v1-1-a7f64b7cfc96@google.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: -ATfGhWdZ_jMwvXTLOThl3wVQfIBhkpplT6YSQHJaMY_1748975609 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: D8FF1180002 X-Stat-Signature: cdpfmqteunyhb4n3d8xx3ccfo8yy3qqc X-Rspam-User: X-HE-Tag: 1748975611-569685 X-HE-Meta: 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 AR6PjhEr KAXQpaXCDE7Yc2DAOLl7cuSNbq0c92KP+lMdDe2OG8C9XifYGNAgONyQ3VhgC4LKJ9tUZVSECzj3z1ozJK5QNlPdtO0E9trT9ay8x4M8lURh1WDNXSrSIBzUbedfHa/j5bdxovgVQY5vB+8wcibwXmdqa77aXffhf12xiyBgg9yVyRUMV8JWUgHsgHVbIe0ZjTk7WZdxQInkQ01ho6H7ebJJJMcUWAAfV8r5VxKu+QddbqSo6tmPOQs6kQUrcrsBzlJ8hxfVCITb0JMKrN0htLfFbBqFo/YDMPcRDG8nP2KHsS1RL7QaY7ZMxnLV5V0jAE2h8tSViWpEx+i/V1DfWdjH7YCEXk4Wt0BlqxaecwVpOpuqBKgVNKM+sKDQ4rNy/0psuB+V8soJpiLJhHlBhXCaPHawnO1pnThmy+8kLQIVbToOa1yJSh0d5EiB5//yiCON/1uGVe9tIu48= 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: On 03.06.25 20:21, Jann Horn wrote: > When fork() encounters possibly-pinned pages, those pages are immediately > copied instead of just marking PTEs to make CoW happen later. If the parent > is multithreaded, this can cause the child to see memory contents that are > inconsistent in multiple ways: > > 1. We are copying the contents of a page with a memcpy() while userspace > may be writing to it. This can cause the resulting data in the child to > be inconsistent. > 2. After we've copied this page, future writes to other pages may> continue to be visible to the child while future writes to this page are > no longer visible to the child. > > This means the child could theoretically see incoherent states where > allocator freelists point to objects that are actually in use or stuff like > that. A mitigating factor is that, unless userspace already has a deadlock > bug, userspace can pretty much only observe such issues when fancy lockless > data structures are used (because if another thread was in the middle of > mutating data during fork() and the post-fork child tried to take the mutex > protecting that data, it might wait forever). > Hmm, interesting. > On top of that, this issue is only observable when pages are either > DMA-pinned or appear false-positive-DMA-pinned due to a page having >=1024 > references and the parent process having used DMA-pinning at least once > before. Right. > > Fixes: 70e806e4e645 ("mm: Do early cow for pinned pages during fork() for ptes") > Cc: stable@vger.kernel.org > Signed-off-by: Jann Horn > --- > mm/memory.c | 18 ++++++++++++++++++ > 1 file changed, 18 insertions(+) > > diff --git a/mm/memory.c b/mm/memory.c > index 49199410805c..b406dfda976b 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -917,7 +917,25 @@ copy_present_page(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma > /* > * We have a prealloc page, all good! Take it > * over and copy the page & arm it. > + * > + * One nasty aspect is that we could be in a multithreaded process or > + * such, where another thread is in the middle of writing to memory > + * while this thread is forking. As long as we're just marking PTEs as > + * read-only to make copy-on-write happen *later*, that's easy; we just > + * need to do a single TLB flush before dropping the mmap/VMA locks, and > + * that's enough to guarantee that the child gets a coherent snapshot of > + * memory. > + * But here, where we're doing an immediate copy, we must ensure that > + * threads in the parent process can no longer write into the page being > + * copied until we're done forking. > + * This means that we still need to mark the source PTE as read-only, > + * with an immediate TLB flush. > + * (To make the source PTE writable again after fork() is done, we can > + * rely on the page fault handler to do that lazily, thanks to > + * PageAnonExclusive().) > */ > + ptep_set_wrprotect(src_vma->vm_mm, addr, src_pte); > + flush_tlb_page(src_vma, addr); Would we need something similar for hugetlb, or is that already handled? -- Cheers, David / dhildenb