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

[Serge E. Hallyn]

On Wed, Nov 02, 2022 at 07:24:51PM +0100, Christian Brauner wrote:
> On Mon, Sep 05, 2022 at 05:30:36PM +0200, Christian Brauner wrote:
> > 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 at kernel.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.
> 
> I have a patch in rough shape that converts struct simple_xattr to use
> an rhashtable:
> 
> https://gitlab.com/brauner/linux/-/commits/fs.xattr.simple.rework/
> 
> Light testing, not a lot useful comments and no meaningful commit
> message as of yet but I'll get to that.
> 
> Even though your issue is orthogonal to the performance issues I'm
> trying to fix I went back to your patch, Ondrej to apply it on top.
> But I think it has one problem.
> 
> Afaict, by moving the capable() call from the top of the function into
> the actual traversal portion an unprivileged user can potentially learn
> whether a file has trusted.* xattrs set. At least if dmesg isn't
> restricted on the kernel. That may very well be the reason why the
> capable() call is on top.
> (Because the straightforward fix for this would be to just call
> capable() a single time if at least one trusted xattr is encountered and
> store the result. That's pretty easy to do by making turning the trusted
> variable into an int, setting it to -1, and only if it's -1 and a
> trusted xattr has been found call capable() and store the result.)
> 
> One option to fix all of that is to switch simple_xattr_list() to use
> 
>         ns_capable_noaudit(&init_user_ns, CAP_SYS_ADMIN)
> 
> which doesn't generate an audit event.
> 
> I think this is even the correct thing to do as listing xattrs isn't a
> targeted operation. IOW, if the the user had used getxattr() to request
> a trusted.* xattr then logging a denial makes sense as the user
> explicitly wanted to retrieve a trusted.* xattr. But if the user just
> requested to list all xattrs then silently skipping trusted without
> logging an explicit denial xattrs makes sense.
> 
> Does that sound acceptable?

Agreed, auditing that seems like unwanted noise.