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

[Eric Biggers]

On Wed, Jul 11, 2018 at 08:22:41AM +0100, David Howells wrote:
> Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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?  Youâre okay as long as *every single fs* gets it right, but thatâs
> > 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