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[2003:cb:c72f:c700:a76f:473d:d5cf:25a8]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-42f86a20595sm117161735e9.14.2024.10.08.00.50.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 08 Oct 2024 00:50:15 -0700 (PDT) Message-ID: <316dadcb-b199-4b94-8d07-94b40bf534df@redhat.com> Date: Tue, 8 Oct 2024 09:50:14 +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/mremap: Fix move_normal_pmd/retract_page_tables race To: Jann Horn , akpm@linux-foundation.org Cc: linux-mm@kvack.org, willy@infradead.org, hughd@google.com, lorenzo.stoakes@oracle.com, joel@joelfernandes.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20241007-move_normal_pmd-vs-collapse-fix-2-v1-1-5ead9631f2ea@google.com> Content-Language: en-US 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: <20241007-move_normal_pmd-vs-collapse-fix-2-v1-1-5ead9631f2ea@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 07.10.24 23:42, Jann Horn wrote: > In mremap(), move_page_tables() looks at the type of the PMD entry and the > specified address range to figure out by which method the next chunk of > page table entries should be moved. > At that point, the mmap_lock is held in write mode, but no rmap locks are > held yet. For PMD entries that point to page tables and are fully covered > by the source address range, move_pgt_entry(NORMAL_PMD, ...) is called, > which first takes rmap locks, then does move_normal_pmd(). > move_normal_pmd() takes the necessary page table locks at source and > destination, then moves an entire page table from the source to the > destination. > > The problem is: The rmap locks, which protect against concurrent page table > removal by retract_page_tables() in the THP code, are only taken after the > PMD entry has been read and it has been decided how to move it. > So we can race as follows (with two processes that have mappings of the > same tmpfs file that is stored on a tmpfs mount with huge=advise); note > that process A accesses page tables through the MM while process B does it > through the file rmap: > > > process A process B > ========= ========= > mremap > mremap_to > move_vma > move_page_tables > get_old_pmd > alloc_new_pmd > *** PREEMPT *** > madvise(MADV_COLLAPSE) > do_madvise > madvise_walk_vmas > madvise_vma_behavior > madvise_collapse > hpage_collapse_scan_file > collapse_file > retract_page_tables > i_mmap_lock_read(mapping) > pmdp_collapse_flush > i_mmap_unlock_read(mapping) > move_pgt_entry(NORMAL_PMD, ...) > take_rmap_locks > move_normal_pmd > drop_rmap_locks > > When this happens, move_normal_pmd() can end up creating bogus PMD entries > in the line `pmd_populate(mm, new_pmd, pmd_pgtable(pmd))`. > The effect depends on arch-specific and machine-specific details; on x86, > you can end up with physical page 0 mapped as a page table, which is likely > exploitable for user->kernel privilege escalation. > > > Fix the race by letting process B recheck that the PMD still points to a > page table after the rmap locks have been taken. Otherwise, we bail and let > the caller fall back to the PTE-level copying path, which will then bail > immediately at the pmd_none() check. > > Bug reachability: Reaching this bug requires that you can create shmem/file > THP mappings - anonymous THP uses different code that doesn't zap stuff > under rmap locks. File THP is gated on an experimental config flag > (CONFIG_READ_ONLY_THP_FOR_FS), so on normal distro kernels you need shmem > THP to hit this bug. As far as I know, getting shmem THP normally requires > that you can mount your own tmpfs with the right mount flags, which would > require creating your own user+mount namespace; though I don't know if some > distros maybe enable shmem THP by default or something like that. > > Bug impact: This issue can likely be used for user->kernel privilege > escalation when it is reachable. > > Cc: stable@vger.kernel.org > Fixes: 1d65b771bc08 ("mm/khugepaged: retract_page_tables() without mmap or vma lock") > Closes: https://project-zero.issues.chromium.org/371047675 > Co-developed-by: David Hildenbrand > Signed-off-by: Jann Horn > --- > @David: please confirm we can add your Signed-off-by to this patch after > the Co-developed-by. Sure, thanks for sending this out! Signed-off-by: David Hildenbrand -- Cheers, David / dhildenb