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

[Al Viro]

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

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.
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