On Tue 11-04-23 16:01:16, Christian Brauner wrote: > On Tue, Apr 11, 2023 at 03:40:25PM +0200, Christian Brauner wrote: > > On Tue, Apr 11, 2023 at 12:55:42PM +0200, Jan Kara wrote: > > > On Tue 11-04-23 12:11:52, Christian Brauner wrote: > > > > On Mon, Apr 10, 2023 at 11:53:46PM -0700, syzbot wrote: > > > > > Hello, > > > > > > > > > > syzbot found the following issue on: > > > > > > > > > > HEAD commit: 0d3eb744aed4 Merge tag 'urgent-rcu.2023.04.07a' of git://g.. > > > > > git tree: upstream > > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=11798e4bc80000 > > > > > kernel config: https://syzkaller.appspot.com/x/.config?x=c21559e740385326 > > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=cdcd444e4d3a256ada13 > > > > > compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2 > > > > > > > > > > Unfortunately, I don't have any reproducer for this issue yet. > > > > > > > > > > Downloadable assets: > > > > > disk image: https://storage.googleapis.com/syzbot-assets/a02928003efa/disk-0d3eb744.raw.xz > > > > > vmlinux: https://storage.googleapis.com/syzbot-assets/7839447005a4/vmlinux-0d3eb744.xz > > > > > kernel image: https://storage.googleapis.com/syzbot-assets/d26ab3184148/bzImage-0d3eb744.xz > > > > > > > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit: > > > > > Reported-by: syzbot+cdcd444e4d3a256ada13@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > > > > > > > ====================================================== > > > > > WARNING: possible circular locking dependency detected > > > > > 6.3.0-rc6-syzkaller-00016-g0d3eb744aed4 #0 Not tainted > > > > > ------------------------------------------------------ > > > > > syz-executor.3/11858 is trying to acquire lock: > > > > > ffff88802a3bc0e0 (&type->s_umount_key#31){++++}-{3:3}, at: __do_sys_quotactl_fd+0x174/0x3f0 fs/quota/quota.c:997 > > > > > > > > > > but task is already holding lock: > > > > > ffff88802a3bc460 (sb_writers#4){.+.+}-{0:0}, at: __do_sys_quotactl_fd+0xd3/0x3f0 fs/quota/quota.c:990 > > > > > > > > > > which lock already depends on the new lock. > > > > > > > > > > > > > > > the existing dependency chain (in reverse order) is: > > > > > > > > > > -> #1 (sb_writers#4){.+.+}-{0:0}: > > > > > percpu_down_read include/linux/percpu-rwsem.h:51 [inline] > > > > > __sb_start_write include/linux/fs.h:1477 [inline] > > > > > sb_start_write include/linux/fs.h:1552 [inline] > > > > > write_mmp_block+0xc4/0x820 fs/ext4/mmp.c:50 > > > > > ext4_multi_mount_protect+0x50d/0xac0 fs/ext4/mmp.c:343 > > > > > __ext4_remount fs/ext4/super.c:6543 [inline] > > > > > ext4_reconfigure+0x242b/0x2b60 fs/ext4/super.c:6642 > > > > > reconfigure_super+0x40c/0xa30 fs/super.c:956 > > > > > vfs_fsconfig_locked fs/fsopen.c:254 [inline] > > > > > __do_sys_fsconfig+0xa3a/0xc20 fs/fsopen.c:439 > > > > > do_syscall_x64 arch/x86/entry/common.c:50 [inline] > > > > > do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80 > > > > > entry_SYSCALL_64_after_hwframe+0x63/0xcd > > > > > > > > > > -> #0 (&type->s_umount_key#31){++++}-{3:3}: > > > > > check_prev_add kernel/locking/lockdep.c:3098 [inline] > > > > > check_prevs_add kernel/locking/lockdep.c:3217 [inline] > > > > > validate_chain kernel/locking/lockdep.c:3832 [inline] > > > > > __lock_acquire+0x2ec7/0x5d40 kernel/locking/lockdep.c:5056 > > > > > lock_acquire kernel/locking/lockdep.c:5669 [inline] > > > > > lock_acquire+0x1af/0x520 kernel/locking/lockdep.c:5634 > > > > > down_write+0x92/0x200 kernel/locking/rwsem.c:1573 > > > > > __do_sys_quotactl_fd+0x174/0x3f0 fs/quota/quota.c:997 > > > > > do_syscall_x64 arch/x86/entry/common.c:50 [inline] > > > > > do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80 > > > > > entry_SYSCALL_64_after_hwframe+0x63/0xcd > > > > > > > > > > other info that might help us debug this: > > > > > > > > > > Possible unsafe locking scenario: > > > > > > > > > > CPU0 CPU1 > > > > > ---- ---- > > > > > lock(sb_writers#4); > > > > > lock(&type->s_umount_key#31); > > > > > lock(sb_writers#4); > > > > > lock(&type->s_umount_key#31); > > > > > > > > > > *** DEADLOCK *** > > > > > > > > Hmkay, I understand how this happens, I think: > > > > > > > > fsconfig(FSCONFIG_CMD_RECONFIGURE) quotactl_fd(Q_QUOTAON/Q_QUOTAOFF/Q_XQUOTAON/Q_XQUOTAOFF) > > > > -> mnt_want_write(f.file->f_path.mnt); > > > > -> down_write(&sb->s_umount); -> __sb_start_write(sb, SB_FREEZE_WRITE) > > > > -> reconfigure_super(fc); > > > > -> ext4_multi_mount_protect() > > > > -> __sb_start_write(sb, SB_FREEZE_WRITE) -> down_write(&sb->s_umount); > > > > -> up_write(&sb->s_umount); > > > > > > Thanks for having a look! > > > > > > > I have to step away from the computer now for a bit but naively it seem > > > > that the locking order for quotactl_fd() should be the other way around. > > > > > > > > But while I'm here, why does quotactl_fd() take mnt_want_write() but > > > > quotactl() doesn't? It seems that if one needs to take it both need to > > > > take it. > > > > > > Couple of notes here: > > > > > > 1) quotactl() handles the filesystem freezing by grabbing the s_umount > > > semaphore, checking the superblock freeze state (it cannot change while > > > s_umount is held) and proceeding if fs is not frozen. This logic is hidden > > > in quotactl_block(). > > > > > > 2) The proper lock ordering is indeed freeze-protection -> s_umount because > > > that is implicitely dictated by how filesystem freezing works. If you grab > > > > Yep. > > One final thought about this. quotactl() and quotactl_fd() could do the > same thing though, right? quotactl() could just be made to use the same > locking scheme as quotactl_fd(). Not saying it has to, but the code > would probably be easier to understand/maintain if both would use the same. Yes, that would be nice. But quotactl(2) gets a block device as an argument, needs to translate that to a superblock (user_get_super()) and only then we could use sb_start_write() to protect from fs freezing - but we already hold s_umount from user_get_super() so we can't do that due to lock ordering. That's why handling the freeze protection is so contrived in quotactl(2). We used to have variant of user_get_super() that guaranteed returning thawed superblock but Christoph didn't like it and only quota code was using it so stuff got opencoded in the quota code instead (see commit 60b498852bf2 ("fs: remove get_super_thawed and get_super_exclusive_thawed"). Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
