linux-kernel - Re: [PATCH 0/2] fs: fix capable() call in simple_xattr

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Message-ID: <20220905153036.zzcovknz7ntgcn5f@wittgenstein>
Date:   Mon, 5 Sep 2022 17:30:36 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Ondrej Mosnacek <omosnace@...hat.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux Security Module list 
        <linux-security-module@...r.kernel.org>,
        SElinux list <selinux@...r.kernel.org>, rcu@...r.kernel.org,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>,
        Martin Pitt <mpitt@...hat.com>, Vasily Averin <vvs@...nvz.org>
Subject: Re: [PATCH 0/2] fs: fix capable() call in simple_xattr_list()

On Mon, Sep 05, 2022 at 12:15:01PM +0200, Ondrej Mosnacek wrote:
> On Mon, Sep 5, 2022 at 11:08 AM Christian Brauner <brauner@...nel.org> 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@openvz.org
> >
> > 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

The rfc that Vasily sent didn't allow for that at least.

> 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.

I've never used the rhashtable infrastructure in any meaningful way. All
I can say from looking at current users that it looks like it could work
well for us here:

struct simple_xattr {
	struct rhlist_head rhlist_head;
	char *name;
	size_t size;
	char value[];
};

static const struct rhashtable_params simple_xattr_rhashtable = {
	.head_offset = offsetof(struct simple_xattr, rhlist_head),
	.key_offset = offsetof(struct simple_xattr, name),

or sm like this.