[systemd-devel] Antw: [EXT] Re: Creating executable device nodes in /dev?

[Jarkko Sakkinen]

On Mon, Dec 14, 2020 at 08:25:50AM +0100, Ulrich Windl wrote:
> >>> Topi Miettinen <toiwoton at gmail.com> schrieb am 11.12.2020 um 12:46 in
> Nachricht
> <27796c04-249e-6cf0-c3e1-0fd657a82f9c at gmail.com>:
> > On 11.12.2020 12.46, Jarkko Sakkinen wrote:
> >> On Wed, Dec 09, 2020 at 10:35:21AM +0200, Topi Miettinen wrote:
> >>> On 9.12.2020 2.15, Jarkko Sakkinen wrote:
> >>>> On Wed, Dec 09, 2020 at 01:15:27AM +0200, Topi Miettinen wrote:
> >>>>>>>> As  a further argument, I just did this on a Fedora system:
> >>>>>>>> $ find /dev ‑perm /ugo+x ‑a \! ‑type d ‑a \! ‑type l
> >>>>>>>> No results.  So making /dev noexec doesn't seem to have any benefit.
> >>>>>>>
> >>>>>>> It's no surprise that there aren't any executables in /dev since
> >>>>>>> removing MAKEDEV ages ago. That's not the issue, which is that
> >>>>>>> /dev is a writable directory (for UID=0 but no capabilities are
> >>>>>>> needed) and thus a potential location for constructing unapproved
> >>>>>>> executables if it is also mounted exec (W^X).
> >>>>>>
> >>>>>> UID 0 can just change mount options, though, unless SELinux or similar
> is 
> > used. And SELinux can protect /dev just fine without noexec.
> >>>>>
> >>>>> Well, mounting would need CAP_SYS_ADMIN in addition to UID 0. Also
> SELinux
> >>>>> is not universal and the policies might not contain all users or
> services.
> >>>>>
> >>>>> ‑Topi
> >>>>
> >>>> What's the data that supports having noexec /dev anyway? With root
> >>>> access I can then just use something else like /dev/shm mount.
> >>>>
> >>>> Has there been out in the wild real world cases that noexec mount
> >>>> of would have prevented?
> >>>>
> >>>> For me this sounds a lot just something that "feels more secure"
> >>>> without any measurable benefit. Can you prove me wrong?
> >>>
> >>> I don't think security works that way. An attacker has various methods to
> >>> choose from, some are more interesting than others. The case where
> rw,exec
> >>> /dev would be interesting would imply that easier or more common avenues
> >>> would be blocked, for example rw,exec /dev/shm, /tmp, /var/tmp, or
> >>> /run/user/$UID/ for user. Also fileless malware with pure ROP/JOP
> approach
> >>> with no need for any file system access is getting more common. It does
> not
> >>> mean that it would not be prudent to block the relatively easy approaches
> >>> too, including /dev.
> >> 
> >> What if we add a new mount option "chrexec", which allows exec
> >> for character devices (S_IFCHR).
> > 
> > I think devices are a bad match for SGX because devices haven't been 
> > executable and SGX is actually an operation for memory. So something 
> > like memfd_create(, MFD_SGX) or mmap(,, PROT_READ|PROT_EXEC|PROT_SGX) 
> > would be much more natural. Even better would be something that 
> > conceptully also works for AMD version (either with the same flags or 
> > MFD_SGX / MFD_whatever_the_AMD_version_is).
> 
> +1

SGX reserved memory from kernel's point of view is IO memory.

Mapping SGX to memfd would not be a great idea, as it does not map
into concept of anonymous file backed by regular memory.

A device file is very natural match actually. We have ioctl API for
uploading enclave pages during the build procedure to the enclave and
custom #PF handler. Conceptually it's a lot like video memory or such
special device specific memory area.

There's no AMD equivalent of this technology.

/Jarkko