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[2003:cb:c70e:6700:cada:5396:a4f8:1434]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-39efa420740sm2413742f8f.16.2025.04.18.03.42.43 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 18 Apr 2025 03:42:43 -0700 (PDT) Message-ID: Date: Fri, 18 Apr 2025 12:42:42 +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 v2] mm/huge_memory: fix dereferencing invalid pmd migration entry To: Gavin Guo , linux-mm@kvack.org, akpm@linux-foundation.org Cc: willy@infradead.org, ziy@nvidia.com, linmiaohe@huawei.com, hughd@google.com, revest@google.com, kernel-dev@igalia.com, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20250418085802.2973519-1-gavinguo@igalia.com> 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/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: <20250418085802.2973519-1-gavinguo@igalia.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 18.04.25 10:58, Gavin Guo wrote: > When migrating a THP, concurrent access to the PMD migration entry > during a deferred split scan can lead to a invalid address access, as > illustrated below. To prevent this page fault, it is necessary to check > the PMD migration entry and return early. In this context, there is no > need to use pmd_to_swp_entry and pfn_swap_entry_to_page to verify the > equality of the target folio. Since the PMD migration entry is locked, > it cannot be served as the target. > > Mailing list discussion and explanation from Hugh Dickins: > "An anon_vma lookup points to a location which may contain the folio of > interest, but might instead contain another folio: and weeding out those > other folios is precisely what the "folio != pmd_folio((*pmd)" check > (and the "risk of replacing the wrong folio" comment a few lines above > it) is for." > > BUG: unable to handle page fault for address: ffffea60001db008 > CPU: 0 UID: 0 PID: 2199114 Comm: tee Not tainted 6.14.0+ #4 NONE > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 > RIP: 0010:split_huge_pmd_locked+0x3b5/0x2b60 > Call Trace: > > try_to_migrate_one+0x28c/0x3730 > rmap_walk_anon+0x4f6/0x770 > unmap_folio+0x196/0x1f0 > split_huge_page_to_list_to_order+0x9f6/0x1560 > deferred_split_scan+0xac5/0x12a0 > shrinker_debugfs_scan_write+0x376/0x470 > full_proxy_write+0x15c/0x220 > vfs_write+0x2fc/0xcb0 > ksys_write+0x146/0x250 > do_syscall_64+0x6a/0x120 > entry_SYSCALL_64_after_hwframe+0x76/0x7e > > The bug is found by syzkaller on an internal kernel, then confirmed on > upstream. > > Fixes: 84c3fc4e9c56 ("mm: thp: check pmd migration entry in common path") > Cc: stable@vger.kernel.org > Signed-off-by: Gavin Guo > Acked-by: David Hildenbrand > Acked-by: Hugh Dickins > Acked-by: Zi Yan > Link: https://lore.kernel.org/all/20250414072737.1698513-1-gavinguo@igalia.com/ > --- > V1 -> V2: Add explanation from Hugh and correct the wording from page > fault to invalid address access. > > mm/huge_memory.c | 18 ++++++++++++++---- > 1 file changed, 14 insertions(+), 4 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 2a47682d1ab7..0cb9547dcff2 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -3075,6 +3075,8 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, > void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address, > pmd_t *pmd, bool freeze, struct folio *folio) > { > + bool pmd_migration = is_pmd_migration_entry(*pmd); > + > VM_WARN_ON_ONCE(folio && !folio_test_pmd_mappable(folio)); > VM_WARN_ON_ONCE(!IS_ALIGNED(address, HPAGE_PMD_SIZE)); > VM_WARN_ON_ONCE(folio && !folio_test_locked(folio)); > @@ -3085,10 +3087,18 @@ void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address, > * require a folio to check the PMD against. Otherwise, there > * is a risk of replacing the wrong folio. > */ > - if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) || > - is_pmd_migration_entry(*pmd)) { > - if (folio && folio != pmd_folio(*pmd)) > - return; > + if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) || pmd_migration) { > + if (folio) { > + /* > + * Do not apply pmd_folio() to a migration entry; and > + * folio lock guarantees that it must be of the wrong > + * folio anyway. > + */ > + if (pmd_migration) > + return; > + if (folio != pmd_folio(*pmd)) > + return; Nit: just re-reading, I would have simply done if (pmd_migration || folio != pmd_folio(*pmd) return; Anyway, this will hopefully get cleaned up soon either way, so I don't particularly mind. :) -- Cheers, David / dhildenb