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

[Topi Miettinen]

On 16.12.2020 12.03, Ulrich Windl wrote:
>>>> Jarkko Sakkinen <jarkko at kernel.org> schrieb am 15.12.2020 um 05:19 in
> Nachricht
> <20201215041903.GA21875 at kernel.org>:
>> 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.
> 
> Hi!
> 
> Back to "noexec": AFAIR the execute bit does not make sense for device files,
> and the purpose probably was to avoid execution of non-device files (e.g.
> regular executables) from inside /dev (where they should not be). So in this
> view "noexec" makes sense.
> There were similar arguments for not allowing device files in user
> directories.

PR#17940 (https://github.com/systemd/systemd/pull/17940) was merged, so 
/dev will now on be mounted with "exec" by systemd.

I made issue #17942 (https://github.com/systemd/systemd/issues/17942) to 
discuss related hardening options. I'm leaning towards 
NoExecPaths=/ExecPaths= as it would enable nice hardening by 
allow-listing of all executable content for system services with simple 
directives like:

[Service]
NoExecPaths=/
ExecPaths=/usr/sbin/daemon /lib64/ld-linux-x86-64.so.2 /usr/lib

Then a service infected with malware would not be able to execute a 
shell present in the system or downloaded later, if that was not 
explicitly allowed. /dev would also not have "exec" flag by default, but 
SGX could be allowed with "ExecPaths=/dev/sgx" when needed.

-Topi