Re: [syzbot] [xfs?] WARNING in fsnotify_file_area_perm

[Josef Bacik <josef@toxicpanda.com>]

On Tue, Mar 04, 2025 at 04:09:16PM +0100, Amir Goldstein wrote:
> On Tue, Mar 4, 2025 at 12:06 PM Jan Kara <jack@suse.cz> wrote:
> >
> > Josef, Amir,
> >
> > this is indeed an interesting case:
> >
> > On Sun 02-03-25 08:32:30, syzbot wrote:
> > > syzbot has found a reproducer for the following issue on:
> > ...
> > > ------------[ cut here ]------------
> > > WARNING: CPU: 1 PID: 6440 at ./include/linux/fsnotify.h:145 fsnotify_file_area_perm+0x20c/0x25c include/linux/fsnotify.h:145
> > > Modules linked in:
> > > CPU: 1 UID: 0 PID: 6440 Comm: syz-executor370 Not tainted 6.14.0-rc4-syzkaller-ge056da87c780 #0
> > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 12/27/2024
> > > pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> > > pc : fsnotify_file_area_perm+0x20c/0x25c include/linux/fsnotify.h:145
> > > lr : fsnotify_file_area_perm+0x20c/0x25c include/linux/fsnotify.h:145
> > > sp : ffff8000a42569d0
> > > x29: ffff8000a42569d0 x28: ffff0000dcec1b48 x27: ffff0000d68a1708
> > > x26: ffff0000d68a16c0 x25: dfff800000000000 x24: 0000000000008000
> > > x23: 0000000000000001 x22: ffff8000a4256b00 x21: 0000000000001000
> > > x20: 0000000000000010 x19: ffff0000d68a16c0 x18: ffff8000a42566e0
> > > x17: 000000000000e388 x16: ffff800080466c24 x15: 0000000000000001
> > > x14: 1fffe0001b31513c x13: 0000000000000000 x12: 0000000000000000
> > > x11: 0000000000000001 x10: 0000000000ff0100 x9 : 0000000000000000
> > > x8 : ffff0000c6d98000 x7 : 0000000000000000 x6 : 0000000000000000
> > > x5 : 0000000000000020 x4 : 0000000000000000 x3 : 0000000000001000
> > > x2 : ffff8000a4256b00 x1 : 0000000000000001 x0 : 0000000000000000
> > > Call trace:
> > >  fsnotify_file_area_perm+0x20c/0x25c include/linux/fsnotify.h:145 (P)
> > >  filemap_fault+0x12b0/0x1518 mm/filemap.c:3509
> > >  xfs_filemap_fault+0xc4/0x194 fs/xfs/xfs_file.c:1543
> > >  __do_fault+0xf8/0x498 mm/memory.c:4988
> > >  do_read_fault mm/memory.c:5403 [inline]
> > >  do_fault mm/memory.c:5537 [inline]
> > >  do_pte_missing mm/memory.c:4058 [inline]
> > >  handle_pte_fault+0x3504/0x57b0 mm/memory.c:5900
> > >  __handle_mm_fault mm/memory.c:6043 [inline]
> > >  handle_mm_fault+0xfa8/0x188c mm/memory.c:6212
> > >  do_page_fault+0x570/0x10a8 arch/arm64/mm/fault.c:690
> > >  do_translation_fault+0xc4/0x114 arch/arm64/mm/fault.c:783
> > >  do_mem_abort+0x74/0x200 arch/arm64/mm/fault.c:919
> > >  el1_abort+0x3c/0x5c arch/arm64/kernel/entry-common.c:432
> > >  el1h_64_sync_handler+0x60/0xcc arch/arm64/kernel/entry-common.c:510
> > >  el1h_64_sync+0x6c/0x70 arch/arm64/kernel/entry.S:595
> > >  __uaccess_mask_ptr arch/arm64/include/asm/uaccess.h:169 [inline] (P)
> > >  fault_in_readable+0x168/0x310 mm/gup.c:2234 (P)
> > >  fault_in_iov_iter_readable+0x1dc/0x22c lib/iov_iter.c:94
> > >  iomap_write_iter fs/iomap/buffered-io.c:950 [inline]
> > >  iomap_file_buffered_write+0x490/0xd54 fs/iomap/buffered-io.c:1039
> > >  xfs_file_buffered_write+0x2dc/0xac8 fs/xfs/xfs_file.c:792
> > >  xfs_file_write_iter+0x2c4/0x6ac fs/xfs/xfs_file.c:881
> > >  new_sync_write fs/read_write.c:586 [inline]
> > >  vfs_write+0x704/0xa9c fs/read_write.c:679
> >
> > The backtrace actually explains it all. We had a buffered write whose
> > buffer was mmapped file on a filesystem with an HSM mark. Now the prefaulting
> > of the buffer happens already (quite deep) under the filesystem freeze
> > protection (obtained in vfs_write()) which breaks assumptions of HSM code
> > and introduces potential deadlock of HSM handler in userspace with filesystem
> > freezing. So we need to think how to deal with this case...
> 
> Ouch. It's like the splice mess all over again.
> Except we do not really care to make this use case work with HSM
> in the sense that we do not care to have to fill in the mmaped file content
> in this corner case - we just need to let HSM fail the access if content is
> not available.
> 
> If you remember, in one of my very early version of pre-content events,
> the pre-content event (or maybe it was FAN_ACCESS_PERM itself)
> carried a flag (I think it was called FAN_PRE_VFS) to communicate to
> HSM service if it was safe to write to fs in the context of event handling.
> 
> At the moment, I cannot think of any elegant way out of this use case
> except annotating the event from fault_in_readable() as "unsafe-for-write".
> This will relax the debugging code assertion and notify the HSM service
> (via an event flag) that it can ALLOW/DENY, but it cannot fill the file.
> Maybe we can reuse the FAN_ACCESS_PERM event to communicate
> this case to HSM service.
> 
> WDYT?

I think that mmap was a mistake.

Is there a way to tell if we're currently in a path that is under fsfreeze
protection?  Just denying this case would be a simpler short term solution while
we come up with a long term solution. I think your solution is fine, but I'd be
just as happy with a simpler "this isn't allowed" solution. Thanks,

Josef