linux-kernel - Re: [kernfs] e19dfdc83b: BUG:KASAN:global-out-of-bounds_in

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Date:   Tue, 26 Mar 2019 13:33:54 +0100
From:   Ondrej Mosnacek <omosnace@...hat.com>
To:     Paul Moore <paul@...l-moore.com>
Cc:     Casey Schaufler <casey@...aufler-ca.com>,
        LKML <linux-kernel@...r.kernel.org>, selinux@...r.kernel.org,
        lkp@...org, kernel test robot <rong.a.chen@...el.com>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [kernfs] e19dfdc83b: BUG:KASAN:global-out-of-bounds_in_s

On Mon, Mar 25, 2019 at 6:06 PM Ondrej Mosnacek <omosnace@...hat.com> wrote:
> On Mon, Mar 25, 2019 at 4:17 PM Paul Moore <paul@...l-moore.com> wrote:
> > Ondrej, please look into this.
> >
> > You've looked at this code more recently than I have, but it looks
> > like there might be an issue with __kernfs_iattrs() returning a
> > pointer to a kernfs_iattrs object without taking a kernfs reference
> > (kernfs_get(kn)).  Although I would be a little surprised if this was
> > the problem as I think it would cause a number of issues beyond just
> > this one ... ?
>
> I think this is actually because of how xattr_full_name() reconstructs
> the full name from the xattr suffix. It assumes that the suffix was
> obtained from the full name by just taking a pointer inside it, but in
> kernfs_security_xattr_get/set() I pass the suffix directly... I'm
> surprised that this didn't fail spectacularly earlier during testing.
> Maybe the newer GCC does some clever merging of the string constants,
> so that XATTR_SELINUX_SUFFIX actually ends up as a substring of
> XATTR_NAME_SELINUX? (That would be one hell of a "lucky" coincidence
> :)
>
> I'll post a patch that converts kernfs_security_xattr_get/set() to
> take the full name and hopefully that will fix the problem. I'll see
> if I can run the reproducer locally tomorrow...

I managed to reproduce the KASAN warning in my kernel testing
environment by simply enabling CONFIG_KASAN and running the cgroupfs
issue reproducer from the original patchset. With the patch I posted I
no longer get the warning, so I believe it really fixes the problem.

--
Ondrej Mosnacek <omosnace at redhat dot com>
Software Engineer, Security Technologies
Red Hat, Inc.