linux-kernel - Re: security: lockless stat() issues

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Message-ID: <CA+55aFzt1LN77WgFbdEak0f-JssQH64pSL=1zYAfB7=B8pxwvQ@mail.gmail.com>
Date:	Fri, 4 Oct 2013 13:49:13 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	James Morris <jmorris@...ei.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	LSM List <linux-security-module@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: security: lockless stat() issues

On Fri, Oct 4, 2013 at 1:15 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> Just a couple of days ago you'd been complaining about filesystems exposure
> to rcuwalk details and now you propose to increase the contact surface
> by one more method?  OK...\

.. anything for a really critical path.

> For one thing, that will definitely require lazy freeing of struct
> super_block itself.  At the very least, you want ->i_sb->s_dev, even
> on default pathway.  Another thing is that API is wrong for that
> kind of stuff - we pass vfsmount/dentry pair and we end up looking
> at the inode guts.  Fine, but in RCU case you must cope with the
> possibility of dentry->d_inode going NULL under you.  Which isn't
> all that terrible, but we need to slap ACCESS_ONCE() into each method
> instance (all 34 of them).  IOW, it's worse than just "oh, we need to
> add a flag"; we'll need to pass the sodding inode separately, with
> dire warnings not to use dentry->d_inode.

That would get ugly.

However, I don't think we actually really need to do that.  We had a
similar situation with d_revalidate() passing inode pointers etc
totally unnecessarily. Yes, the filesystem needs to use ACCESS_ONCE()
and care about NULL, but it doesn't need anything more than that. And
we really do have that already.

And we already have dentry->d_sb - which is supposed to be valid.
Again, we already use it under RCU for d_revalidate() and for name
hashing. So the super-block had better already be ok with RCU.

            Linus
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