linux-kernel - Re: [PATCH v2 04/10] ovl: make ioctl() safe

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Date:   Thu, 10 Dec 2020 16:19:19 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     James Morris <jmorris@...ei.org>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        linux-fsdevel@...r.kernel.org,
        overlayfs <linux-unionfs@...r.kernel.org>,
        LSM <linux-security-module@...r.kernel.org>,
        linux-kernel@...r.kernel.org, Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v2 04/10] ovl: make ioctl() safe

On Wed, Dec 9, 2020 at 3:01 AM James Morris <jmorris@...ei.org> wrote:
>
> On Mon, 7 Dec 2020, Miklos Szeredi wrote:
>
> > ovl_ioctl_set_flags() does a capability check using flags, but then the
> > real ioctl double-fetches flags and uses potentially different value.
> >
> > The "Check the capability before cred override" comment misleading: user
> > can skip this check by presenting benign flags first and then overwriting
> > them to non-benign flags.
>
> Is this a security bug which should be fixed in stable?

Yes, good point.  Added Cc: stable@...

Thanks,
Miklos