Re: [PATCH 0/2] fs: fix capable() call in simple_xattr_list()

[Ondrej Mosnacek]

On Mon, Sep 5, 2022 at 11:08 AM Christian Brauner <brauner@xxxxxxxxxx> wrote:
> On Thu, Sep 01, 2022 at 05:26:30PM +0200, Ondrej Mosnacek wrote:
> > The goal of these patches is to avoid calling capable() unconditionally
> > in simple_xattr_list(), which causes issues under SELinux (see
> > explanation in the second patch).
> >
> > The first patch tries to make this change safer by converting
> > simple_xattrs to use the RCU mechanism, so that capable() is not called
> > while the xattrs->lock is held. I didn't find evidence that this is an
> > issue in the current code, but it can't hurt to make that change
> > either way (and it was quite straightforward).
>
> Hey Ondrey,
>
> There's another patchset I'd like to see first which switches from a
> linked list to an rbtree to get rid of performance issues in this code
> that can be used to dos tmpfs in containers:
>
> https://lore.kernel.org/lkml/d73bd478-e373-f759-2acb-2777f6bba06f@xxxxxxxxxx
>
> I don't think Vasily has time to continue with this so I'll just pick it
> up hopefully this or the week after LPC.

Hm... does rbtree support lockless traversal? Because if not, that
would make it impossible to fix the issue without calling capable()
inside the critical section (or doing something complicated), AFAICT.
Would rhashtable be a workable alternative to rbtree for this use
case? Skimming <linux/rhashtable.h> it seems to support both lockless
lookup and traversal using RCU. And according to its manpage,
*listxattr(2) doesn't guarantee that the returned names are sorted.

--
Ondrej Mosnacek
Senior Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.