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

[Ulrich Windl]

>>> 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