Date:   Tue, 7 Dec 2021 13:39:06 -0800
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        syzbot <syzbot+7cd473c2cac13fd2dd72@...kaller.appspotmail.com>,
        linux-mm@...ck.org, syzkaller-bugs@...glegroups.com,
        linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [syzbot] BUG: unable to handle kernel NULL pointer
 dereference in folio_mark_dirty

On 12/07, Matthew Wilcox wrote:
> On Mon, Dec 06, 2021 at 05:56:31PM -0800, Andrew Morton wrote:
> > On Sat, 04 Dec 2021 01:55:17 -0800 syzbot <syzbot+7cd473c2cac13fd2dd72@...kaller.appspotmail.com> wrote:
> > 
> > > Hello,
> > > 
> > > syzbot found the following issue on:
> > > 
> > > HEAD commit:    58e1100fdc59 MAINTAINERS: co-maintain random.c
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=1362881eb00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=e9ea28d2c3c2c389
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=7cd473c2cac13fd2dd72
> > > compiler:       Debian clang version 11.0.1-2, GNU ld (GNU Binutils for Debian) 2.35.2
> > > 
> > > Unfortunately, I don't have any reproducer for this issue yet.
> > > 
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+7cd473c2cac13fd2dd72@...kaller.appspotmail.com
> > > 
> > > BUG: kernel NULL pointer dereference, address: 0000000000000000
> > 
> > cc linux-f2fs-devel@...ts.sourceforge.net
> > 
> > And willy, who might help with diagnosing this.  But it does seem that
> > f2fs got itself a NULL page* then put it in places where it shouldn't have.
> 
> Oh -- it's not a NULL data pointer.  It's a NULL instruction pointer.
> 
> > > #PF: supervisor instruction fetch in kernel mode
> > > #PF: error_code(0x0010) - not-present page
> > > PGD 70764067 P4D 70764067 PUD 0 
> > > Oops: 0010 [#1] PREEMPT SMP KASAN
> > > CPU: 1 PID: 6541 Comm: syz-executor.3 Not tainted 5.16.0-rc3-syzkaller #0
> > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > > RIP: 0010:0x0
> > > Code: Unable to access opcode bytes at RIP 0xffffffffffffffd6.
> > > RSP: 0018:ffffc900027ff7f8 EFLAGS: 00010246
> > > RAX: 1ffffffff14fef03 RBX: ffffffff8a7f7818 RCX: ffff88801b40d700
> > > RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffea0002790ec0
> > > RBP: dffffc0000000000 R08: ffffffff81b0fa16 R09: fffff940004f21d9
> > > R10: fffff940004f21d9 R11: 0000000000000000 R12: ffff88806c11c7b0
> > > R13: 0000000000000000 R14: 1ffffd40004f21d9 R15: ffffea0002790ec0
> > > FS:  0000555557165400(0000) GS:ffff8880b9b00000(0000) knlGS:0000000000000000
> > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > CR2: ffffffffffffffd6 CR3: 0000000030d85000 CR4: 00000000003526e0
> > > Call Trace:
> > >  <TASK>
> > >  folio_mark_dirty+0x136/0x270 mm/page-writeback.c:2639
> 
>         if (likely(mapping)) {
> ...
>                 if (folio_test_reclaim(folio))
>                         folio_clear_reclaim(folio);
>                 return mapping->a_ops->set_page_dirty(&folio->page);
> 
> how do we get to a NULL ->set_page_dirty for a metadata page's
> mapping->a_ops?  This is definitely an f2fs expert question.

I can't find anything in f2fs, since that page was got by f2fs_grab_meta_page
along with grab_cache_page() that we never unlocked it.

  40 struct page *f2fs_grab_meta_page(struct f2fs_sb_info *sbi, pgoff_t index)
  41 {
  42         struct address_space *mapping = META_MAPPING(sbi);
  43         struct page *page;
  44 repeat:
  45         page = f2fs_grab_cache_page(mapping, index, false);

                    -> grab_cache_page(mapping, index);

  46         if (!page) {
  47                 cond_resched();
  48                 goto repeat;
  49         }
  50         f2fs_wait_on_page_writeback(page, META, true, true);
  51         if (!PageUptodate(page))
  52                 SetPageUptodate(page);
  53         return page;
  54 }


Suspecting something in folio wrt folio_mapping()?

 81 bool set_page_dirty(struct page *page)
 82 {
 83         return folio_mark_dirty(page_folio(page));
 84 }


> 
> > >  f2fs_update_meta_page+0x4b/0x380 fs/f2fs/segment.c:2485
> > >  do_checkpoint fs/f2fs/checkpoint.c:1513 [inline]
> > >  f2fs_write_checkpoint+0x31ad/0x5430 fs/f2fs/checkpoint.c:1674
> > >  f2fs_issue_checkpoint+0x361/0x4e0
> > >  sync_filesystem+0x19c/0x1f0 fs/sync.c:63
> > >  generic_shutdown_super+0x6b/0x300 fs/super.c:448
> > >  kill_block_super+0x79/0xd0 fs/super.c:1397
> > >  kill_f2fs_super+0x2f9/0x3c0 fs/f2fs/super.c:4478
> > >  deactivate_locked_super+0xa7/0xf0 fs/super.c:335
> > >  cleanup_mnt+0x462/0x510 fs/namespace.c:1137
> > >  task_work_run+0x146/0x1c0 kernel/task_work.c:164
> > >  tracehook_notify_resume include/linux/tracehook.h:189 [inline]
> > >  exit_to_user_mode_loop kernel/entry/common.c:175 [inline]
> > >  exit_to_user_mode_prepare+0x209/0x220 kernel/entry/common.c:207
> > >  __syscall_exit_to_user_mode_work kernel/entry/common.c:289 [inline]
> > >  syscall_exit_to_user_mode+0x2e/0x70 kernel/entry/common.c:300
> > >  do_syscall_64+0x53/0xd0 arch/x86/entry/common.c:86
> > >  entry_SYSCALL_64_after_hwframe+0x44/0xae
> > > RIP: 0033:0x7f6cfdd59f57
> > > Code: ff ff ff f7 d8 64 89 01 48 83 c8 ff c3 66 0f 1f 44 00 00 31 f6 e9 09 00 00 00 66 0f 1f 84 00 00 00 00 00 b8 a6 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 bc ff ff ff f7 d8 64 89 01 48
> > > RSP: 002b:00007fffcbddcad8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6
> > > RAX: 0000000000000000 RBX: 0000000000000000 RCX: 00007f6cfdd59f57
> > > RDX: 00007fffcbddcbac RSI: 000000000000000a RDI: 00007fffcbddcba0
> > > RBP: 00007fffcbddcba0 R08: 00000000ffffffff R09: 00007fffcbddc970
> > > R10: 0000555557166903 R11: 0000000000000246 R12: 00007f6cfddb2105
> > > R13: 00007fffcbdddc60 R14: 0000555557166810 R15: 00007fffcbdddca0
> > >  </TASK>
> > > Modules linked in:
> > > CR2: 0000000000000000
> > > ---[ end trace 08eda5a5e35b48a0 ]---
> > > RIP: 0010:0x0
> > > Code: Unable to access opcode bytes at RIP 0xffffffffffffffd6.
> > > RSP: 0018:ffffc900027ff7f8 EFLAGS: 00010246
> > > RAX: 1ffffffff14fef03 RBX: ffffffff8a7f7818 RCX: ffff88801b40d700
> > > RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffea0002790ec0
> > > RBP: dffffc0000000000 R08: ffffffff81b0fa16 R09: fffff940004f21d9
> > > R10: fffff940004f21d9 R11: 0000000000000000 R12: ffff88806c11c7b0
> > > R13: 0000000000000000 R14: 1ffffd40004f21d9 R15: ffffea0002790ec0
> > > FS:  0000555557165400(0000) GS:ffff8880b9b00000(0000) knlGS:0000000000000000
> > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > CR2: ffffffffffffffd6 CR3: 0000000030d85000 CR4: 00000000003526e0
> > > 
> > > 
> > > ---
> > > 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.
> 
> 
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