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Message-ID: <CA+EESO7goQHuAdobDvWMaYGiVw30g1=o_f5UzBceeSbS2n5MtA@mail.gmail.com> Date: Tue, 29 Jul 2025 01:08:00 -0700 From: Lokesh Gidra <lokeshgidra@...gle.com> To: Suren Baghdasaryan <surenb@...gle.com> Cc: Peter Xu <peterx@...hat.com>, syzbot <syzbot+b446dbe27035ef6bd6c2@...kaller.appspotmail.com>, akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org, syzkaller-bugs@...glegroups.com Subject: Re: [syzbot] [mm?] BUG: unable to handle kernel paging request in move_pages On Mon, Jul 28, 2025 at 7:51 PM Suren Baghdasaryan <surenb@...gle.com> wrote: > > On Mon, Jul 28, 2025 at 9:08 PM Peter Xu <peterx@...hat.com> wrote: > > > > Copy Lokesh and Suren. Thanks Peter! > > Thanks! I'll take a closer look tomorrow morning. > I think the issue is that we are incorrectly handling src holes in the THP case. The reproducer is setting 'mode' to UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES and it seems like the src address is indeed untouched at the time MOVE ioctl is invoked and hence likely has a hole. When this mode is set, we (correctly) don't fail with -ENOENT, but then instead of skipping the page, we keep going with THP move, which involves fetching the folio unconditionally from the src_pmd, which is expected to have no page mapped there. Suren, can you please double check if my hypothesis is correct? > > > > On Thu, Jul 17, 2025 at 12:13:32PM -0700, syzbot wrote: > > > Hello, > > > > > > syzbot found the following issue on: > > > > > > HEAD commit: e8352908bdcd Add linux-next specific files for 20250716 > > > git tree: linux-next > > > console+strace: https://syzkaller.appspot.com/x/log.txt?x=17f81382580000 > > > kernel config: https://syzkaller.appspot.com/x/.config?x=b7b0e60e17dc5717 > > > dashboard link: https://syzkaller.appspot.com/bug?extid=b446dbe27035ef6bd6c2 > > > compiler: Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7 > > > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=10041382580000 > > > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=10eb158c580000 > > > > > > Downloadable assets: > > > disk image: https://storage.googleapis.com/syzbot-assets/ae8cc81c1781/disk-e8352908.raw.xz > > > vmlinux: https://storage.googleapis.com/syzbot-assets/57aaea991896/vmlinux-e8352908.xz > > > kernel image: https://storage.googleapis.com/syzbot-assets/feb871619bd4/bzImage-e8352908.xz > > > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit: > > > Reported-by: syzbot+b446dbe27035ef6bd6c2@...kaller.appspotmail.com > > > > > > BUG: unable to handle page fault for address: ffffea6000391008 > > > #PF: supervisor read access in kernel mode > > > #PF: error_code(0x0000) - not-present page > > > PGD 13fff8067 P4D 13fff8067 PUD 0 > > > Oops: Oops: 0000 [#1] SMP KASAN PTI > > > CPU: 1 UID: 0 PID: 5860 Comm: syz-executor832 Not tainted 6.16.0-rc6-next-20250716-syzkaller #0 PREEMPT(full) > > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025 > > > RIP: 0010:_compound_head include/linux/page-flags.h:284 [inline] > > > RIP: 0010:move_pages+0xbe6/0x1430 mm/userfaultfd.c:1824 > > > Code: c1 ec 06 4b 8d 1c 2c 48 83 c3 08 48 89 d8 48 c1 e8 03 48 b9 00 00 00 00 00 fc ff df 80 3c 08 00 74 08 48 89 df e8 9a 30 f4 ff <48> 8b 1b 48 89 de 48 83 e6 01 31 ff e8 59 70 8f ff 48 89 d8 48 83 > > > RSP: 0018:ffffc90003f778a8 EFLAGS: 00010246 > > > RAX: 1ffffd4c00072201 RBX: ffffea6000391008 RCX: dffffc0000000000 > > > RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000 > > > RBP: 0000000000000000 R08: 0000000000000003 R09: 0000000000000004 > > > R10: dffffc0000000000 R11: fffff520007eef00 R12: 0000006000391000 > > > R13: ffffea0000000000 R14: 200018000e4401fd R15: 00002000003ab000 > > > FS: 00007ff35708f6c0(0000) GS:ffff8881258aa000(0000) knlGS:0000000000000000 > > > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > > > CR2: ffffea6000391008 CR3: 0000000074390000 CR4: 00000000003526f0 > > > Call Trace: > > > <TASK> > > > userfaultfd_move fs/userfaultfd.c:1923 [inline] > > > userfaultfd_ioctl+0x2e8b/0x4c80 fs/userfaultfd.c:2046 > > > vfs_ioctl fs/ioctl.c:51 [inline] > > > __do_sys_ioctl fs/ioctl.c:598 [inline] > > > __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:584 > > > do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline] > > > do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94 > > > entry_SYSCALL_64_after_hwframe+0x77/0x7f > > > RIP: 0033:0x7ff3570d6519 > > > Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 51 18 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 b0 ff ff ff f7 d8 64 89 01 48 > > > RSP: 002b:00007ff35708f218 EFLAGS: 00000246 ORIG_RAX: 0000000000000010 > > > RAX: ffffffffffffffda RBX: 00007ff357160308 RCX: 00007ff3570d6519 > > > RDX: 0000200000000180 RSI: 00000000c028aa05 RDI: 0000000000000003 > > > RBP: 00007ff357160300 R08: 0000000000000000 R09: 0000000000000000 > > > R10: 0000000000000000 R11: 0000000000000246 R12: 00007ff35712d074 > > > R13: 0000200000000180 R14: 0000200000000188 R15: 00002000002b9000 > > > </TASK> > > > Modules linked in: > > > CR2: ffffea6000391008 > > > ---[ end trace 0000000000000000 ]--- > > > RIP: 0010:_compound_head include/linux/page-flags.h:284 [inline] > > > RIP: 0010:move_pages+0xbe6/0x1430 mm/userfaultfd.c:1824 > > > Code: c1 ec 06 4b 8d 1c 2c 48 83 c3 08 48 89 d8 48 c1 e8 03 48 b9 00 00 00 00 00 fc ff df 80 3c 08 00 74 08 48 89 df e8 9a 30 f4 ff <48> 8b 1b 48 89 de 48 83 e6 01 31 ff e8 59 70 8f ff 48 89 d8 48 83 > > > RSP: 0018:ffffc90003f778a8 EFLAGS: 00010246 > > > RAX: 1ffffd4c00072201 RBX: ffffea6000391008 RCX: dffffc0000000000 > > > RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000 > > > RBP: 0000000000000000 R08: 0000000000000003 R09: 0000000000000004 > > > R10: dffffc0000000000 R11: fffff520007eef00 R12: 0000006000391000 > > > R13: ffffea0000000000 R14: 200018000e4401fd R15: 00002000003ab000 > > > FS: 00007ff35708f6c0(0000) GS:ffff8881258aa000(0000) knlGS:0000000000000000 > > > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > > > CR2: ffffea6000391008 CR3: 0000000074390000 CR4: 00000000003526f0 > > > ---------------- > > > Code disassembly (best guess): > > > 0: c1 ec 06 shr $0x6,%esp > > > 3: 4b 8d 1c 2c lea (%r12,%r13,1),%rbx > > > 7: 48 83 c3 08 add $0x8,%rbx > > > b: 48 89 d8 mov %rbx,%rax > > > e: 48 c1 e8 03 shr $0x3,%rax > > > 12: 48 b9 00 00 00 00 00 movabs $0xdffffc0000000000,%rcx > > > 19: fc ff df > > > 1c: 80 3c 08 00 cmpb $0x0,(%rax,%rcx,1) > > > 20: 74 08 je 0x2a > > > 22: 48 89 df mov %rbx,%rdi > > > 25: e8 9a 30 f4 ff call 0xfff430c4 > > > * 2a: 48 8b 1b mov (%rbx),%rbx <-- trapping instruction > > > 2d: 48 89 de mov %rbx,%rsi > > > 30: 48 83 e6 01 and $0x1,%rsi > > > 34: 31 ff xor %edi,%edi > > > 36: e8 59 70 8f ff call 0xff8f7094 > > > 3b: 48 89 d8 mov %rbx,%rax > > > 3e: 48 rex.W > > > 3f: 83 .byte 0x83 > > > > > > > > > --- > > > This report is generated by a bot. It may contain errors. > > > See https://goo.gl/tpsmEJ for more information about syzbot. > > > syzbot engineers can be reached at syzkaller@...glegroups.com. > > > > > > syzbot will keep track of this issue. See: > > > https://goo.gl/tpsmEJ#status for how to communicate with syzbot. > > > > > > If the report is already addressed, let syzbot know by replying with: > > > #syz fix: exact-commit-title > > > > > > If you want syzbot to run the reproducer, reply with: > > > #syz test: git://repo/address.git branch-or-commit-hash > > > If you attach or paste a git patch, syzbot will apply it before testing. > > > > > > If you want to overwrite report's subsystems, reply with: > > > #syz set subsystems: new-subsystem > > > (See the list of subsystem names on the web dashboard) > > > > > > If the report is a duplicate of another one, reply with: > > > #syz dup: exact-subject-of-another-report > > > > > > If you want to undo deduplication, reply with: > > > #syz undup > > > > > > > -- > > Peter Xu > >
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