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[2003:d8:2f09:3f00:d228:bd67:7baa:d604]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4293e41496dsm194726705e9.23.2024.08.13.02.28.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 13 Aug 2024 02:28:30 -0700 (PDT) Message-ID: <2fda0d01-e86a-41ce-95ff-4d24cf3cca32@redhat.com> Date: Tue, 13 Aug 2024 11:28:29 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] userfaultfd: Fix pmd_trans_huge() recheck race To: Jann Horn , Andrew Morton , Pavel Emelyanov , Andrea Arcangeli , Hugh Dickins Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20240812-uffd-thp-flip-fix-v1-0-4fc1db7ccdd0@google.com> <20240812-uffd-thp-flip-fix-v1-1-4fc1db7ccdd0@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: <20240812-uffd-thp-flip-fix-v1-1-4fc1db7ccdd0@google.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: yxka3t9t5nm6czn6bit4wg4cop4wr318 X-Rspamd-Queue-Id: 1DFEC140010 X-Rspamd-Server: rspam11 X-HE-Tag: 1723541313-825979 X-HE-Meta: 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 tpiurCfb 31CmlX6KhkhmlzpfrV4Zo5fS/DsboyxWxQGf+KWZbczNexp1pOMQ0viuul821Wz1P3xJap5NwwjhRB2GWD21d/LvEdX1Cjr4nRorrv7tIK/m+Sr84zcAql8X4xy3OPYHKhNlbzoNXSkD/L+Cm9t0C1u4/a19Ifzoq5UZD2Wnpq67xWdoNsb5hL0NzdcRcSD8eeN0he2D+iqJm5DiGJFxaGlDL8nUD5002i3ckEw5WLEsH4rJ2epj0qEryGBxtzODISWXIkzzxY3JqelPSQRmHqj6GBvyorJg8m2GWYIsGkEM/zuFBgbR9bU9LB8KERKMNpb4otYWsCj/ejafOYDzTb5fMBni7VQMeO5048njA5fjIqslqMXda+tY9ls+evix8OsE3zAlHJyEJhIVgp4uZ/odnsS5utzTun3jweiJzr/OTzAbOvo6GyhDE8xkSxOM/4mdWkbLPGCe/0PQ6aMWnFpxG05sNdPuZ7d277CTeMFGuMd9qmUb0RewVyP3YanMe7JPY3DR7qSNcoWM= 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 12.08.24 18:42, Jann Horn wrote: > The following race can occur: > > mfill_atomic other thread > ============ ============ > > pmdp_get_lockless() [reads none pmd] > > > > __pte_alloc [no-op] > > > BUG_ON(pmd_none(*dst_pmd)) > > I have experimentally verified this in a kernel with extra mdelay() calls; > the BUG_ON(pmd_none(*dst_pmd)) triggers. > > On kernels newer than commit 0d940a9b270b ("mm/pgtable: allow > pte_offset_map[_lock]() to fail"), this can't lead to anything worse than > a BUG_ON(), since the page table access helpers are actually designed to > deal with page tables concurrently disappearing; but on older kernels > (<=6.4), I think we could probably theoretically race past the two BUG_ON() > checks and end up treating a hugepage as a page table. > > Cc: stable@vger.kernel.org > Fixes: c1a4de99fada ("userfaultfd: mcopy_atomic|mfill_zeropage: UFFDIO_COPY|UFFDIO_ZEROPAGE preparation") > Signed-off-by: Jann Horn > --- Acked-by: David Hildenbrand -- Cheers, David / dhildenb