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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 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: [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> Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 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