linux-kernel - Re: [PATCH 7/9 updated] vfs: protect remounting superblock read-only

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Date:	Sat, 23 Oct 2010 21:35:16 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	miklos@...redi.hu, dave@...ux.vnet.ibm.com,
	akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/9 updated] vfs: protect remounting superblock read-only

On Sat, 23 Oct 2010, Al Viro wrote:
> On Fri, Oct 22, 2010 at 06:14:01PM +0200, Miklos Szeredi wrote:
> 
> > @@ -1782,6 +1844,14 @@ int do_add_mount(struct vfsmount *newmnt
> >  
> >  	mnt_flags &= ~(MNT_SHARED | MNT_WRITE_HOLD | MNT_INTERNAL);
> 
> Obviously not enough - you've just added a new flag that needs to be
> trimmed from mnt_flags.
> 
> > +	/* Locking is necessary to prevent racing with remount r/o */
> > +	down_read(&newmnt->mnt_sb->s_umount);
> > +	if (newmnt->mnt_sb->s_flags & MS_RDONLY)
> > +		mnt_flags |= MNT_READONLY;
> > +
> > +	newmnt->mnt_flags = mnt_flags;
> > +	up_read(&newmnt->mnt_sb->s_umount);
> 
> FWIW, I really don't like the way you are doing that; what we really need
> there is a per-sb analog of mnt_want_write()/mnt_drop_write().  With
> mnt_want_write() bumping per-sb write count, which would solve all these
> problems quite nicely.
> 
> NOTE: vfsmount being ro and sb being ro are *independent* things;

Yes, except the mount(2) API which doens't quite let them be changed
independently.

>  either
> is enough to deny writes.  Having remount ro + remount rw lose the state
> of other vfsmounts is a Bad Thing(tm).

Hmm.

> 
> Another thing:
> 	"If clone_mnt() happens while mnt_make_readonly() is running, the
> 	cloned mount might have MNT_WRITE_HOLD flag set, which results in
> 	mnt_want_write() spinning forever on this mount."
> actually means
> 	"neither clone_mnt() nor remounts should ever be done without
> namespace_sem held exclusive; if that ever happens, we have a serious
> bug that can lead to any number of bad things happening".
> 
> Do you actually see such places?  If so, that's what needs fixing.

do_remount() takes s_umount, but not namespace_sem.

Thanks,
Miklos
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