Re: Fwd: Remounting ext4 filesystem from ro to rw fails when quotas are enabled

[Jan Kara]

On Thu 08-06-23 00:40:56, Theodore Ts'o wrote:
> On Wed, Jun 07, 2023 at 12:51:26PM +0700, Bagas Sanjaya wrote:
> > I notice a regression report on Bugzilla [1]. Quoting from it:
> > 
> > > Since commit a44be64, remounting a read-only ext4 filesystem to
> > > become read-write fails when quotas are enabled. The mount syscall
> > > returns -EROFS and outputs the following in dmesg:
> 
> Yeah, and I think all we can do is revert this commit:
> 
>     ext4: don't clear SB_RDONLY when remounting r/w until quota is re-enabled
> 
> I think I saw some messages go by about this getting queued for the
> stable kernel; if so, could it please be dropped?

Yeah, for now I'd revert the fix. It needs more thought and it is not like
it is fixing any serious vulnerability or so.

> > > The problem can be traced back to the changes introduced in commit
> > > a44be64. It appears that the issue arises because the SB_RDONLY
> > > bit of the s_flags field is now only cleared after executing the
> > > ext4_enable_quotas function. However, the vfs_setup_quota_inode
> > > function, called by ext4_enable_quotas, checks whether this bit is
> > > set (fs/quota/dquot.c:2331):
> 
> The problem that we're trying to solve is the malicious syzbot
> reproducer is in one thread, twiddling the file system state from r/o
> to rw and back.  In another thread, it's attempt to create files and
> directories.   And occasionally, we're tripping this warning:
> 
> 	WARN_ON_ONCE(dquot_initialize_needed(inode));
> 
> That's because we're racing with the quota getting initialized, and
> the moment we clear the SB_RDONLY bit the thread which is trying to
> create a directory or file will proceed with the operation --- even
> though the quota subsystem hasn't been initialized yet.  That's why
> the patch attempted to move the clearing the SB_RDONLY bit ahead of
> reiniitalization of the quota subsystem.
> 
> Since this is screwing up the ability to remount the file system rw,
> we need to revert this commit, at which point, we'll be able to
> trigger this warning again.
> 
> So how do we fix the warning?  Well, we could just drop the
> WARN_ON_ONCE(); the downside is that when this race gets hit, the
> quota operations to allocate the block and inode will silently become
> a no-op, which means the quota will get out of sync with reality.

Yes, the warning triggering is actually a good thing because it shows us we
have a race where quota can get out of sync with real space usage.

> Alternatively, we could add a call to the beginning to
> ext4_xattr_block_Set():
> 
> 	if (dquot_initialize_needed(inode))
> 		reutrn -EROFS;
> 
> ... on the theory that the only time we should hit this is when there
> is a quota setup racing with inode creation, and it's better that we
> just let the mkdir or open with O_CREAT fail than to succeed, and
> allocate blocks before the quota subsystem has been initialized.  I'm
> not sure how safe this would be on older quota setups (pre-ext4 quota
> feature), since I suspect the race window is a quite a bit bigger if I
> understand correctly how things worked with the legacy quota support.

No, xattr code is just a messenger here. There is a fundamental problem
that once we clear SB_RDONLY in superblock flags, anybody can come up with
any operation modifying the filesystem but until we are finished with
dquot_resume(), this operation can escape proper quota accounting.

> The final really hacky thing I could imagine is to hack
> dquot_initialize() to something like this:
> 
> int dquot_initialize(struct inode *inode)
> {
> 	ret = __dquot_initialize(inode, -1);
> 	if (ret)
> 		return ret;
> 	if (dquot_initialize_needed(inode)) {
> 		msleep(1000)
> 		return __dquot_initialize(inode, -1);
> 	}
> 	return 0;
> }
> 
> But I threw up a little in my mouth while writing it....
> 
> So I'm tempted to just remove the WARN_ON's, and just accept that if
> superuser wants to twiddle the read/only state of a file system with
> quota, at high rates, while something else is trying to create
> files/directories, most of which will fail while the file system is
> read-only, then the quota may gets out of sync, and... ¯\_(ツ)_/¯
> 
> Since this is only something that crazy people or malicious Syzbot
> reproducers would try to do, I'm really having a hard time bringing
> myself to care.  Especially since we have plenty of other places where
> we aren't doing the dquot_initialize_needed() check, so the
> opportunities for the quota to get out of sync already exist in other
> code paths.
> 
> Jan, what do you think?

Well, I think we should fix this properly. The problem is that currently
changing read-only state is just flipping the flag but we need an
intermediate state like "superblock is really RW but for userspace it is
RO" so that we can prepare the filesystem for taking the writes - in fact
we've already faced very similar problem with orphan replay but there we've
worked around it by forbidding orphan replay on remount and playing a bit
hacky games on mount.

I think I can see a way to solve this problem. I'll send patches.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR