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 09:38:35 -0700
From:   Eric Biggers <ebiggers3@...il.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Andy Lutomirski <luto@...capital.net>, viro@...iv.linux.org.uk,
        linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        jannh@...gle.com
Subject: Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for
 superblock creation [ver #9]

On Wed, Jul 11, 2018 at 08:22:41AM +0100, David Howells 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().
> 
> David

Not really, by default an unprivileged user can still do:

	unshare(CLONE_NEWUSER|CLONE_NEWNS);
	syscall(__NR_fsopen, "ext4", 0);

- Eric