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

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Date:   Mon, 14 Dec 2020 07:44:08 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v2 04/10] ovl: make ioctl() safe

On Thu, Dec 10, 2020 at 5:18 PM Miklos Szeredi <miklos@...redi.hu> wrote:
>
> On Tue, Dec 8, 2020 at 12:15 PM Amir Goldstein <amir73il@...il.com> wrote:
> >
> > On Mon, Dec 7, 2020 at 6:36 PM Miklos Szeredi <mszeredi@...hat.com> 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.
> > >
> > > Just remove the cred override for now, hoping this doesn't cause a
> > > regression.
> > >
> > > The proper solution is to create a new setxflags i_op (patches are in the
> > > works).
> > >
> > > Xfstests don't show a regression.
> > >
> > > Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
> > > Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
> >
> > Looks reasonable
> >
> > Reviewed-by: Amir Goldstein <amir73il@...il.com>
> >
> > > ---
> > >  fs/overlayfs/file.c | 75 ++-------------------------------------------
> > >  1 file changed, 3 insertions(+), 72 deletions(-)
> > >
> > > diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> > > index efccb7c1f9bc..3cd1590f2030 100644
> > > --- a/fs/overlayfs/file.c
> > > +++ b/fs/overlayfs/file.c
> > > @@ -541,46 +541,26 @@ static long ovl_real_ioctl(struct file *file, unsigned int cmd,
> > >                            unsigned long arg)
> > >  {
> > >         struct fd real;
> > > -       const struct cred *old_cred;
> > >         long ret;
> > >
> > >         ret = ovl_real_fdget(file, &real);
> > >         if (ret)
> > >                 return ret;
> > >
> > > -       old_cred = ovl_override_creds(file_inode(file)->i_sb);
> > >         ret = security_file_ioctl(real.file, cmd, arg);
> > >         if (!ret)
> > >                 ret = vfs_ioctl(real.file, cmd, arg);
> > > -       revert_creds(old_cred);
> > >
> > >         fdput(real);
> > >
> > >         return ret;
> > >  }
> > >
> >
> >
> > I wonder if we shouldn't leave a comment behind to explain
> > that no override is intentional.
>
> Comment added.
>
> > I also wonder if "Permission model" sections shouldn't be saying
> > something about ioctl() (current task checks only)? or we just treat
> > this is a breakage of the permission model that needs to be fixed?
>
> This is a breakage of the permission model.  But I don't think this is
> a serious breakage, or one that actually matters.
>

Perhaps, but there is a much bigger issue with this change IMO.
Not because of dropping rule (b) of the permission model, but because
of relaxing rule (a).

Should overlayfs respect the conservative interpretation as it partly did
until this commit, a lower file must not lose IMMUTABLE/APPEND_ONLY
after copy up, but that is exactly what is going to happen if we first
copy up and then fail permission check on setting the flags.

It's true that before this change, file could still be copied up and system
crash would leave it without the flags, but after the change it is much
worse as the flags completely lose their meaning on lower files when
any unprivileged process can remove them.

So I suggest that you undo all the changes except for the no override.

And this calls for a fork of generic/545 to overlay test with lower files.

Thanks,
Amir.