linux-kernel - Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for superblock creation [ver #9]

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Date:   Wed, 11 Jul 2018 10:06:39 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Linux API <linux-api@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for
 superblock creation [ver #9]

> On Jul 11, 2018, at 12:22 AM, David Howells <dhowells@...hat.com> wrote:
>
> Andy Lutomirski <luto@...capital.net> wrote:
>
>>>   sfd = fsopen("ext4", FSOPEN_CLOEXEC);
>>>   write(sfd, "s /dev/sdb1"); // note I'm ignoring write's length arg
>>
>> Imagine some malicious program passes sfd as stdout to a setuid
>> program. That program gets persuaded to write "s /etc/shadow".  What
>> happens?  You’re okay as long as *every single fs* gets it right, but that’s
>> asking a lot.
>
> Do note that you must already have CAP_SYS_ADMIN to be able to call fsopen().

If you’re not allowing it already, someone will want user namespace
root to be able to use this very, very soon.