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[2003:d8:2f4e:fd00:8e13:e3b5:90c8:1159]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a6d145e520sm9837115f8f.20.2025.06.23.10.36.21 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 23 Jun 2025 10:36:22 -0700 (PDT) Message-ID: Date: Mon, 23 Jun 2025 19:36:21 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [syzbot] [mm?] kernel BUG in sanity_check_pinned_pages From: David Hildenbrand To: Pavel Begunkov , Jens Axboe , Alexander Potapenko Cc: syzbot , akpm@linux-foundation.org, catalin.marinas@arm.com, jgg@ziepe.ca, jhubbard@nvidia.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, peterx@redhat.com, syzkaller-bugs@googlegroups.com References: <6857299a.a00a0220.137b3.0085.GAE@google.com> <56862a1d-71c0-4f07-9c1a-9d70069b4d9e@redhat.com> <014a3820-8082-43a6-8bb2-70859cabdbc0@kernel.dk> <6f92b7d6-7d3c-4830-a591-75dc4d55c46c@redhat.com> 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: <6f92b7d6-7d3c-4830-a591-75dc4d55c46c@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: IBXwPY_Jrl-Txv9y3sVIPZKqYtp43cI9vShOLncfQd0_1750700183 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: B778D140011 X-Stat-Signature: kwsn4j714erp93a7gqeuuhz9348y4hta X-Rspam-User: X-HE-Tag: 1750700188-700633 X-HE-Meta: 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 c1BpwQN5 CLJiqWH3+FyrqZyLRdi37UomrYmJN+rM0LBez6ay4O6EGYoHJ0braj0uZnGODYKScerDWs79QSUxCfqUaXro7Qr/c7n8U25frOn1kmflD/bvRTcZ8EFdHDznqbwmoLb/VIlKDV6K0bJqiTLYxdxecmRU1xad0XbdLvnmVVt8Z+j+CV+kshIageEN2231LER6i3DWkl6HPZXZZNYmTHzpVJFKbWNMaoWzxdhNrj29MYAahH3iE+zPs0h6JDRcGgSwEYjjRWpVtKDFh3rA= 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 23.06.25 18:59, David Hildenbrand wrote: > On 23.06.25 18:48, Pavel Begunkov wrote: >> On 6/23/25 16:11, David Hildenbrand wrote: >>> On 23.06.25 16:58, Jens Axboe wrote: >>>> On 6/23/25 6:22 AM, David Hildenbrand wrote: >>>>> On 23.06.25 12:10, David Hildenbrand wrote: >>>>>> On 23.06.25 11:53, Alexander Potapenko wrote: >>>>>>> On Mon, Jun 23, 2025 at 11:29?AM 'David Hildenbrand' via >>>>>>> syzkaller-bugs wrote: >>>>>>>> >> ...>>> When only pinning a single tail page (iovec.iov_len = pagesize), it works as expected. >>>>> >>>>> So, if we pinned two tail pages but end up calling io_release_ubuf()->unpin_user_page() >>>>> on the head page, meaning that "imu->bvec[i].bv_page" points at the wrong folio page >>>>> (IOW, one we never pinned). >>>>> >>>>> So it's related to the io_coalesce_buffer() machinery. >>>>> >>>>> And in fact, in there, we have this weird logic: >>>>> >>>>> /* Store head pages only*/ >>>>> new_array = kvmalloc_array(nr_folios, sizeof(struct page *), GFP_KERNEL); >>>>> ... >>>>> >>>>> >>>>> Essentially discarding the subpage information when coalescing tail pages. >>>>> >>>>> >>>>> I am afraid the whole io_check_coalesce_buffer + io_coalesce_buffer() logic might be >>>>> flawed (we can -- in theory -- coalesc different folio page ranges in >>>>> a GUP result?). >>>>> >>>>> @Jens, not sure if this only triggers a warning when unpinning or if we actually mess up >>>>> imu->bvec[i].bv_page, to end up pointing at (reading/writing) pages we didn't even pin in the first >>>>> place. >>>>> >>>>> Can you look into that, as you are more familiar with the logic? >>>> >>>> Leaving this all quoted and adding Pavel, who wrote that code. I'm >>>> currently away, so can't look into this right now. >> >> Chenliang Li did, but not like it matters >> >>> I did some more digging, but ended up being all confused about io_check_coalesce_buffer() and io_imu_folio_data(). >>> >>> Assuming we pass a bunch of consecutive tail pages that all belong to the same folio, then the loop in io_check_coalesce_buffer() will always >>> run into the >>> >>> if (page_folio(page_array[i]) == folio && >>>     page_array[i] == page_array[i-1] + 1) { >>>     count++; >>>     continue; >>> } >>> >>> case, making the function return "true" ... in io_coalesce_buffer(), we then store the head page ... which seems very wrong. >>> >>> In general, storing head pages when they are not the first page to be coalesced seems wrong. >> >> Yes, it stores the head page even if the range passed to >> pin_user_pages() doesn't cover the head page. > > > It should be converted to unpin_user_folio(), which doesn't seem >> to do sanity_check_pinned_pages(). Do you think that'll be enough >> (conceptually)? Nobody is actually touching the head page in those >> cases apart from the final unpin, and storing the head page is >> more convenient than keeping folios. I'll take a look if it can >> be fully converted to folios w/o extra overhead. > > Assuming we had from GUP > > nr_pages = 2 > pages[0] = folio_page(folio, 1) > pages[1] = folio_page(folio, 2) > > After io_coalesce_buffer() we have > > nr_pages = 1 > pages[0] = folio_page(folio, 0) > > > Using unpin_user_folio() in all places where we could see something like > that would be the right thing to do. The sanity checks are not in > unpin_user_folio() for exactly that reason: we don't know which folio > pages we pinned. > > But now I wonder where you make sure that "Nobody is actually touching > the head page"? > > How do you get back the "which folio range" information after > io_coalesce_buffer() ? > > > If you rely on alignment in virtual address space for you, combined with > imu->folio_shift, that might not work reliably ... FWIW, applying the following on top of origin/master: diff --git a/tools/testing/selftests/mm/cow.c b/tools/testing/selftests/mm/cow.c index dbbcc5eb3dce5..e62a284dcf906 100644 --- a/tools/testing/selftests/mm/cow.c +++ b/tools/testing/selftests/mm/cow.c @@ -946,6 +946,7 @@ static void do_run_with_thp(test_fn fn, enum thp_run thp_run, size_t thpsize) log_test_result(KSFT_FAIL); goto munmap; } + mem = mremap_mem; size = mremap_size; break; case THP_RUN_PARTIAL_SHARED: and then running the selftest, something is not happy: ... # [RUN] R/O-mapping a page registered as iouring fixed buffer ... with partially mremap()'ed THP (512 kB) [34272.021973] Oops: general protection fault, maybe for address 0xffff8bab09d5b000: 0000 [#1] PREEMPT SMP NOPTI [34272.021980] CPU: 3 UID: 0 PID: 1048307 Comm: iou-wrk-1047940 Not tainted 6.14.9-300.fc42.x86_64 #1 [34272.021983] Hardware name: LENOVO 20WNS1F81N/20WNS1F81N, BIOS N35ET53W (1.53 ) 03/22/2023 [34272.021984] RIP: 0010:memcpy+0xc/0x20 [34272.021989] Code: cc cc cc 66 2e 0f 1f 84 00 00 00 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 66 90 48 89 f8 48 89 d1 a4 e9 4d f9 00 00 66 66 2e 0f 1f 84 00 00 00 00 00 66 90 90 90 [34272.021991] RSP: 0018:ffffcff459183c20 EFLAGS: 00010206 [34272.021993] RAX: ffff8bab09d5b000 RBX: 0000000000000fff RCX: 0000000000000fff [34272.021994] RDX: 0000000000000fff RSI: 0021461670800001 RDI: ffff8bab09d5b000 [34272.021995] RBP: ffff8ba794866c40 R08: ffff8bab09d5b000 R09: 0000000000001000 [34272.021996] R10: ffff8ba7a316f9d0 R11: ffff8ba92f133080 R12: 0000000000000fff [34272.021997] R13: ffff8baa85d5b6a0 R14: 0000000000000fff R15: 0000000000001000 [34272.021998] FS: 00007f16c568a740(0000) GS:ffff8baebf580000(0000) knlGS:0000000000000000 [34272.021999] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [34272.022000] CR2: 00007fffb6a10b00 CR3: 00000003df9eb006 CR4: 0000000000f72ef0 [34272.022001] PKRU: 55555554 [34272.022002] Call Trace: [34272.022004] [34272.022005] copy_page_from_iter_atomic+0x36f/0x7e0 [34272.022009] ? simple_xattr_get+0x59/0xa0 [34272.022012] generic_perform_write+0x86/0x2e0 [34272.022016] shmem_file_write_iter+0x86/0x90 [34272.022019] io_write+0xe4/0x390 [34272.022023] io_issue_sqe+0x65/0x4f0 [34272.022024] ? lock_timer_base+0x7d/0xc0 [34272.022027] io_wq_submit_work+0xb8/0x320 [34272.022029] io_worker_handle_work+0xd5/0x300 [34272.022032] io_wq_worker+0xda/0x300 [34272.022034] ? finish_task_switch.isra.0+0x99/0x2c0 [34272.022037] ? __pfx_io_wq_worker+0x10/0x10 [34272.022039] ret_from_fork+0x34/0x50 [34272.022042] ? __pfx_io_wq_worker+0x10/0x10 [34272.022044] ret_from_fork_asm+0x1a/0x30 [34272.022047] There, we essentially mremap a THP to not be aligned in VA space, and then register half the THP as a fixed buffer. So ... my suspicion that this is all rather broken grows :) -- Cheers, David / dhildenb