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

[Ryusuke Konishi]

On Tue, Oct 25, 2022 at 1:49 AM Matthew Wilcox wrote:
>
> Adding the nilfs maintainers ...
>
> On Mon, Oct 24, 2022 at 09:38:40AM -0700, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    bbed346d5a96 Merge branch 'for-next/core' into for-kernelci
> > git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
> > console output: https://syzkaller.appspot.com/x/log.txt?x=15788ec2880000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=3a4a45d2d827c1e
> > dashboard link: https://syzkaller.appspot.com/bug?extid=f1eb7f33bbf683a5e1e1
> > compiler:       Debian clang version 13.0.1-++20220126092033+75e33f71c2da-1~exp1~20220126212112.63, GNU ld (GNU Binutils for Debian) 2.35.2
> > userspace arch: arm64
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/e8e91bc79312/disk-bbed346d.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/c1cb3fb3b77e/vmlinux-bbed346d.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+f1eb7f33bbf683a5e1e1@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > Unable to handle kernel NULL pointer dereference at virtual address 0000000000000050
> > Mem abort info:
> >   ESR = 0x0000000096000005
> >   EC = 0x25: DABT (current EL), IL = 32 bits
> >   SET = 0, FnV = 0
> >   EA = 0, S1PTW = 0
> >   FSC = 0x05: level 1 translation fault
> > Data abort info:
> >   ISV = 0, ISS = 0x00000005
> >   CM = 0, WnR = 0
> > user pgtable: 4k pages, 48-bit VAs, pgdp=000000014a8d0000
> > [0000000000000050] pgd=08000001532c9003, p4d=08000001532c9003, pud=0000000000000000
> > Internal error: Oops: 0000000096000005 [#1] PREEMPT SMP
> > Modules linked in:
> > CPU: 0 PID: 3066 Comm: syz-executor.3 Not tainted 6.0.0-rc7-syzkaller-18095-gbbed346d5a96 #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/30/2022
> > pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> > pc : filemap_free_folio+0x20/0x288 mm/filemap.c:231
> > lr : filemap_free_folio+0x1c/0x288 mm/filemap.c:227
> > sp : ffff80001280b990
> > x29: ffff80001280b990 x28: ffff000117b4ea00 x27: 0000000000000000
> > x26: 0000000000000001 x25: ffff80000cb6fd9e x24: fffffffffffffffe
> > x23: 0000000000000000 x22: fffffc0004cfa8c0 x21: 0000000000000001
> > x20: ffff0001127af450 x19: fffffc0004cfa8c0 x18: 00000000000003b8
> > x17: ffff80000bffd6bc x16: 0000000000000002 x15: 0000000000000000
> > x14: 0000000000000000 x13: 0000000000000003 x12: ffff80000d5f02b0
> > x11: ff808000083c31e8 x10: 0000000000000000 x9 : ffff8000083c31e8
> > x8 : 0000000000000000 x7 : ffff80000856806c x6 : 0000000000000000
> > x5 : 0000000000000080 x4 : 0000000000000000 x3 : 0000000000000000
> > x2 : 0000000000000006 x1 : fffffc0004cfa8c0 x0 : ffff0001127af450
> > Call trace:
> >  filemap_free_folio+0x20/0x288 mm/filemap.c:231
> >  delete_from_page_cache_batch+0x148/0x184 mm/filemap.c:341
> >  truncate_inode_pages_range+0x174/0xb94 mm/truncate.c:370
> >  truncate_inode_pages mm/truncate.c:452 [inline]
> >  truncate_inode_pages_final+0x8c/0x9c mm/truncate.c:487
> >  nilfs_evict_inode+0x58/0x1cc fs/nilfs2/inode.c:906
> >  evict+0xec/0x334 fs/inode.c:665
> >  dispose_list fs/inode.c:698 [inline]
> >  evict_inodes+0x2e0/0x354 fs/inode.c:748
> >  generic_shutdown_super+0x50/0x190 fs/super.c:480
> >  kill_block_super+0x30/0x78 fs/super.c:1427
> >  deactivate_locked_super+0x70/0xe8 fs/super.c:332
> >  deactivate_super+0xd0/0xd4 fs/super.c:363
> >  cleanup_mnt+0x1f8/0x234 fs/namespace.c:1186
> >  __cleanup_mnt+0x20/0x30 fs/namespace.c:1193
> >  task_work_run+0xc4/0x14c kernel/task_work.c:177
> >  resume_user_mode_work include/linux/resume_user_mode.h:49 [inline]
> >  do_notify_resume+0x174/0x1f0 arch/arm64/kernel/signal.c:1127
> >  prepare_exit_to_user_mode arch/arm64/kernel/entry-common.c:137 [inline]
> >  exit_to_user_mode arch/arm64/kernel/entry-common.c:142 [inline]
> >  el0_svc+0x9c/0x150 arch/arm64/kernel/entry-common.c:637
> >  el0t_64_sync_handler+0x84/0xf0 arch/arm64/kernel/entry-common.c:654
> >  el0t_64_sync+0x18c/0x190 arch/arm64/kernel/entry.S:581
> > Code: aa0103f3 aa0003f4 97fb728c f940de88 (f9402914)
> > ---[ end trace 0000000000000000 ]---
> > ----------------
> > Code disassembly (best guess):
> >    0: aa0103f3        mov     x19, x1
> >    4: aa0003f4        mov     x20, x0
> >    8: 97fb728c        bl      0xffffffffffedca38
> >    c: f940de88        ldr     x8, [x20, #440]
> > * 10: f9402914        ldr     x20, [x8, #80] <-- trapping instruction
>
> As far as I can tell, this is:
>
>         free_folio = mapping->a_ops->free_folio;
>
> and the first dereference (mapping->a_ops) is offset 440 from mapping,
> which works fine, but is NULL.  So loading aops->free_folio is the
> NULL pointer dereference.
>
> So does nilfs have an address_space with a NULL a_ops?  That doesn't
> seem to be allowed; at least I don't see any checks of a_ops for
> being NULL in the rest of the VFS or MM.

There is no place where a NULL is set to a_ops in NILFS.
All inodes used by NILFS are allocated by either iget5_locked() or
new_inode(), and both of them at least initialize a_ops to &empty_aops
with inode_init_always().  So, I suspect this is caused by UAF or,
less likely, a memory corruption on inode->i_data.

I'm tracking these possibilities, but haven't been able to identify the cause.
One possibility is that this is a variant of the report bug [1], which
can cause an inode UAF:

[1] https://syzkaller.appspot.com/bug?extid=b8c672b0e22615c80fe0

If so, this is fixed in 6.1-rc1, but not enough evidence yet if it's
the same bug.

Regards,
Ryusuke Konishi