Messages in this thread | | | From | David Howells <> | Subject | Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for superblock creation [ver #9] | Date | Thu, 12 Jul 2018 15:54:04 +0100 |
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Andy Lutomirski <luto@kernel.org> wrote:
> > On Jul 11, 2018, at 12:22 AM, David Howells <dhowells@redhat.com> wrote: > > > > Andy Lutomirski <luto@amacapital.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.
Yeah, I'm sure. And I've been thinking on how to deal with it.
I think we *have* to open the source files/devices with the creds of whoever called fsopen() or fspick() - that way you can't upgrade your privs by passing your context fd to a suid program. To enforce this, I think it's simplest for fscontext_write() to call override_creds() right after taking the uapi_mutex and then call revert_creds() right before dropping the mutex.
Another thing we might want to look at is to allow a supervisory process to examine the context before permitting the create/reconfigure action to proceed. It might also be possible to do this through the LSM.
David
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