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From:   Amir Goldstein <amir73il@gmail.com>
Date:   Mon, 14 Dec 2020 16:47:01 +0200
Message-ID: <CAOQ4uxh-L4dFsbkhHMCacMjLtPimF1OvJgd6uWJP9xYT3rufRA@mail.gmail.com>
Subject: Re: [PATCH v2 04/10] ovl: make ioctl() safe
To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     Miklos Szeredi <mszeredi@redhat.com>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        overlayfs <linux-unionfs@vger.kernel.org>,
        LSM List <linux-security-module@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>
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On Mon, Dec 14, 2020 at 3:24 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> On Mon, Dec 14, 2020 at 6:44 AM Amir Goldstein <amir73il@gmail.com> wrote:
>
> > 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.
>
> Yeah, it's a mess.   This will hopefully sort it out, as it will allow
> easier copy up of flags:
>
> https://lore.kernel.org/linux-fsdevel/20201123141207.GC327006@miu.piliscsaba.redhat.com/
>
> In actual fact losing S_APPEND is not currently prevented by copy-up
> triggered by anything other than FS_IOC_SETX*, and even that is prone
> to races as indicated by the bug report that resulted in this patch.
>
> Let's just fix the IMMUTABLE case:
>
>   - if the file is already copied up with data (since the overlay
> ioctl implementation currently uses the realdata), then we're fine to
> copy up
>
>   - if the file is not IMMUTABLE to start with, then also fine to copy
> up; even if the op will fail after copy up we haven't done anything
> that wouldn't be possible without this particular codepath
>
>   - if task has CAP_LINUX_IMMUTABLE (can add/remove immutable) then
> it's also fine to copy up since we can be fairly sure that the actual
> setflags will succeed as well.  If not, that can be a problem, but as
> I've said copying up IMMUTABLE and other flags should really be done
> by the copy up code, otherwise it won't work properly.
>
> Something like this incremental should be good,  I think:
>
> @@ -576,6 +576,15 @@ static long ovl_ioctl_set_flags(struct f
>
>   inode_lock(inode);
>
> + /*
> + * Prevent copy up if immutable and has no CAP_LINUX_IMMUTABLE
> + * capability.
> + */
> + ret = -EPERM;
> + if (!ovl_has_upperdata(inode) && IS_IMMUTABLE(inode) &&
> +     !capable(CAP_LINUX_IMMUTABLE))
> + goto unlock;
> +
>   ret = ovl_maybe_copy_up(file_dentry(file), O_WRONLY);
>   if (ret)
>   goto unlock;
>

I guess that looks ok for a band aid.

Thanks,
Amir.