On Mon, Sep 5, 2022 at 11:08 AM Christian Brauner [off-list ref] wrote:

On Thu, Sep 01, 2022 at 05:26:30PM +0200, Ondrej Mosnacek wrote:

quoted

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@openvz.org (local)

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.