Re: [syzbot] BUG: unable to handle kernel NULL pointer dereference in folio_mark_dirty

[Matthew Wilcox]

On Mon, Dec 06, 2021 at 05:56:31PM -0800, Andrew Morton wrote:
> > BUG: kernel NULL pointer dereference, address: 0000000000000000
> 
> cc linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx
> 
> 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.

I'm surprised it got that far.

void f2fs_update_meta_page(struct f2fs_sb_info *sbi,
                                        void *src, block_t blk_addr)
{
        struct page *page = f2fs_grab_meta_page(sbi, blk_addr);

        memcpy(page_address(page), src, PAGE_SIZE);
        set_page_dirty(page);
        f2fs_put_page(page, 1);
}

How does page_address(NULL) not crash first?  Or return an address that
can be the target of a memcpy()?

> > #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
> >  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
> > 
> > 
> > ---
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