On 26.01.22 12:29, Jann Horn wrote: > On Wed, Jan 26, 2022 at 11:51 AM David Hildenbrand <david@xxxxxxxxxx> wrote: >> On 20.01.22 21:28, Yang Shi wrote: >>> The syzbot reported the below BUG: >>> >>> kernel BUG at include/linux/page-flags.h:785! >>> invalid opcode: 0000 [#1] PREEMPT SMP KASAN >>> CPU: 1 PID: 4392 Comm: syz-executor560 Not tainted 5.16.0-rc6-syzkaller #0 >>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 >>> RIP: 0010:PageDoubleMap include/linux/page-flags.h:785 [inline] >>> RIP: 0010:__page_mapcount+0x2d2/0x350 mm/util.c:744 >>> Code: e8 d3 16 d1 ff 48 c7 c6 c0 00 b6 89 48 89 ef e8 94 4e 04 00 0f 0b e8 bd 16 d1 ff 48 c7 c6 60 01 b6 89 48 89 ef e8 7e 4e 04 00 <0f> 0b e8 a7 16 d1 ff 48 c7 c6 a0 01 b6 89 4c 89 f7 e8 68 4e 04 00 >>> RSP: 0018:ffffc90002b6f7b8 EFLAGS: 00010293 >>> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000 >>> RDX: ffff888019619d00 RSI: ffffffff81a68c12 RDI: 0000000000000003 >>> RBP: ffffea0001bdc2c0 R08: 0000000000000029 R09: 00000000ffffffff >>> R10: ffffffff8903e29f R11: 00000000ffffffff R12: 00000000ffffffff >>> R13: 00000000ffffea00 R14: ffffc90002b6fb30 R15: ffffea0001bd8001 >>> FS: 00007faa2aefd700(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000 >>> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >>> CR2: 00007fff7e663318 CR3: 0000000018c6e000 CR4: 00000000003506e0 >>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 >>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 >>> Call Trace: >>> <TASK> >>> page_mapcount include/linux/mm.h:837 [inline] >>> smaps_account+0x470/0xb10 fs/proc/task_mmu.c:466 >>> smaps_pte_entry fs/proc/task_mmu.c:538 [inline] >>> smaps_pte_range+0x611/0x1250 fs/proc/task_mmu.c:601 >>> walk_pmd_range mm/pagewalk.c:128 [inline] >>> walk_pud_range mm/pagewalk.c:205 [inline] >>> walk_p4d_range mm/pagewalk.c:240 [inline] >>> walk_pgd_range mm/pagewalk.c:277 [inline] >>> __walk_page_range+0xe23/0x1ea0 mm/pagewalk.c:379 >>> walk_page_vma+0x277/0x350 mm/pagewalk.c:530 >>> smap_gather_stats.part.0+0x148/0x260 fs/proc/task_mmu.c:768 >>> smap_gather_stats fs/proc/task_mmu.c:741 [inline] >>> show_smap+0xc6/0x440 fs/proc/task_mmu.c:822 >>> seq_read_iter+0xbb0/0x1240 fs/seq_file.c:272 >>> seq_read+0x3e0/0x5b0 fs/seq_file.c:162 >>> vfs_read+0x1b5/0x600 fs/read_write.c:479 >>> ksys_read+0x12d/0x250 fs/read_write.c:619 >>> do_syscall_x64 arch/x86/entry/common.c:50 [inline] >>> do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80 >>> entry_SYSCALL_64_after_hwframe+0x44/0xae >>> RIP: 0033:0x7faa2af6c969 >>> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 11 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48 >>> RSP: 002b:00007faa2aefd288 EFLAGS: 00000246 ORIG_RAX: 0000000000000000 >>> RAX: ffffffffffffffda RBX: 00007faa2aff4418 RCX: 00007faa2af6c969 >>> RDX: 0000000000002025 RSI: 0000000020000100 RDI: 0000000000000003 >>> RBP: 00007faa2aff4410 R08: 00007faa2aefd700 R09: 0000000000000000 >>> R10: 00007faa2aefd700 R11: 0000000000000246 R12: 00007faa2afc20ac >>> R13: 00007fff7e6632bf R14: 00007faa2aefd400 R15: 0000000000022000 >>> </TASK> >>> Modules linked in: >>> ---[ end trace 24ec93ff95e4ac3d ]--- >>> RIP: 0010:PageDoubleMap include/linux/page-flags.h:785 [inline] >>> RIP: 0010:__page_mapcount+0x2d2/0x350 mm/util.c:744 >>> Code: e8 d3 16 d1 ff 48 c7 c6 c0 00 b6 89 48 89 ef e8 94 4e 04 00 0f 0b e8 bd 16 d1 ff 48 c7 c6 60 01 b6 89 48 89 ef e8 7e 4e 04 00 <0f> 0b e8 a7 16 d1 ff 48 c7 c6 a0 01 b6 89 4c 89 f7 e8 68 4e 04 00 >>> RSP: 0018:ffffc90002b6f7b8 EFLAGS: 00010293 >>> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000 >>> RDX: ffff888019619d00 RSI: ffffffff81a68c12 RDI: 0000000000000003 >>> RBP: ffffea0001bdc2c0 R08: 0000000000000029 R09: 00000000ffffffff >>> R10: ffffffff8903e29f R11: 00000000ffffffff R12: 00000000ffffffff >>> R13: 00000000ffffea00 R14: ffffc90002b6fb30 R15: ffffea0001bd8001 >>> FS: 00007faa2aefd700(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000 >>> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >>> CR2: 00007fff7e663318 CR3: 0000000018c6e000 CR4: 00000000003506e0 >>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 >>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 >>> >> >> Does this point at the bigger issue that reading the mapcount without >> having the page locked is completely unstable? > > (See also https://lore.kernel.org/all/CAG48ez0M=iwJu=Q8yUQHD-+eZDg6ZF8QCF86Sb=CN1petP=Y0Q@xxxxxxxxxxxxxx/ > for context.) Thanks for the pointer. > > I'm not sure what you mean by "unstable". Do you mean "the result is > not guaranteed to still be valid when the call returns", "the result > might not have ever been valid", or "the call might crash because the > page's state as a compound page is unstable"? A little bit of everything :) > > In case you mean "the result is not guaranteed to still be valid when > the call returns": > We're just collecting stats for userspace, and by the time we return > to userspace, the numbers will be outdated anyway, so that doesn't > matter. Yes, it's a snapshot. > > In case you mean "the result might not have ever been valid": > Yes, even with this patch applied, in theory concurrent THP splits > could cause us to count some page mappings twice. Arguably that's not > entirely correct. Yes, the snapshot is not atomic and, thereby, unreliable. That what I mostly meant as "unstable". > > In case you mean "the call might crash because the page's state as a > compound page could concurrently change": I think that's just a side-product of the snapshot not being "correct", right? > As long as we have our own mapping of the page, the page can't be > split, so this patch fixes that problem. -- Thanks, David / dhildenb
