linux-kernel - Re: [PATCH v3 1/1] ovl: setxattr: don't deadlock when called from ima_fix

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Date:	Fri, 20 May 2016 17:29:37 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
Cc:	Krisztian Litkey <kli@....fi>, linux-unionfs@...r.kernel.org,
	Krisztian Litkey <krisztian.litkey@...el.com>,
	linux-security-module <linux-security-module@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/1] ovl: setxattr: don't deadlock when called from
 ima_fix_xattr.

On Fri, May 20, 2016 at 10:21:27AM -0400, Mimi Zohar wrote:
> > +	if (mutex_is_locked(&upper->d_inode->i_mutex))
> > +		err = __vfs_setxattr_noperm(upper, name, value, size, flags);
> 
> As far as I'm aware, the only time that i_mutex is taken, is during
> __fput() when IMA writes security.ima.   Previous versions of this patch
> checked whether the xattr being written was security.ima.  It would
> probably be a good idea not to make that assumption here.   The question
> is what should happen if the i_mutex is locked, but the xattr isn't
> security.ima.  At minimum it should be audited.  Al, any comments?

ITYM "printable", and that's somewhat harder.  OK, let me try:

Anybody using ..._is_lock() kind of primitives that way ought to be
(re)educated before they are allowed near any kind of multithreaded
code _anywhere_.  mutex could've been held by a different thread of
execution and dropped just as mutex_is_locked() returns.  Or at any
subsequent point.  This is 100% bogus; one should *never* write that kind
of code.  As in "here's your well-earned F-, better luck next semester".

I haven't seen the full patch (you've quoted only a part of that gem), but
about the only way for it to be correct is to have it continue with
+ else
+       err = <identical call>

	Practically all calls of mutex_is_locked() are of form
WARN_ON(!mutex_is_locked(...)) or equivalent thereof.  And the rest contains
similar... wonders - for example, take a look at drivers/media/rc/imon.c;
imon_ir_change_protocol() has this
        if (!mutex_is_locked(&ictx->lock)) {
                unlock = true;
                mutex_lock(&ictx->lock);
        }

        retval = send_packet(ictx);
        if (retval)
                goto out;

        ictx->rc_type = *rc_type;
        ictx->pad_mouse = false;

out:
        if (unlock)
                mutex_unlock(&ictx->lock);
Finding why it's exploitably racy is left as a trivial exercise for readers...

Folks, if you see something of that sort in the code, it's a huge red flag.
There are legitimate uses of mutex_is_locked other than asserts, but those
are extremely rare.

I would need to see more context to be able to comment on the problem in
question, but this patch is almost certainly broken.