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

[Christian Brauner <brauner@kernel.org>]

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@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?