Re: Creating executable device nodes in /dev?

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On 2020-12-09 09:58, Topi Miettinen wrote:
> On 9.12.2020 2.42, Jarkko Sakkinen wrote:
>> On Wed, Dec 09, 2020 at 02:15:28AM +0200, Jarkko Sakkinen wrote:
>>> On Wed, Dec 09, 2020 at 01:15:27AM +0200, Topi Miettinen wrote:
>>>>>>> As=C2=A0 a further argument, I just did this on a Fedora system:
>>>>>>> $ find /dev -perm /ugo+x -a \! -type d -a \! -type l
>>>>>>> No results.=C2=A0 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=3D0 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 simi=
lar is used. And SELinux can protect /dev just fine without noexec.
>>>>
>>>> Well, mounting would need CAP_SYS_ADMIN in addition to UID 0. Also S=
ELinux
>>>> is not universal and the policies might not contain all users or ser=
vices.
>>>>
>>>> -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?
>>
>> Typo: "of" =3D "of /dev"
>>
>>> For me this sounds a lot just something that "feels more secure"
>>> without any measurable benefit. Can you prove me wrong?
>>
>> The debate is circled around something not well defined. Of course you=

>> get theoretically more safe system when you decrease priviliges anywhe=
re
>> in the system. Like you could start do grazy things with stuff that
>> unprivilged user has access, in order to prevent malware to elevate to=

>> UID 0 in the first place.
>>
>> I think where this go intellectually wrong is that we are talking abou=
t
>> *default installation* of a distribution. That should have somewhat sa=
ne
>> common sense access control settings. For like a normal desktop user
>> noexec /dev will not do any possible favor.
>>
>> Then there is the case when you want to harden installation for an
>> application, let's' say some server. In that case you will anyway
>> fine-tune the security settings and go grazy enough with hardening.
>> When you tailor a server, it's a standard practice to enumerate and
>> adjust the mount points if needed.
>=20
> I think we agree that there's a need for either way to allow SGX (if de=
fault is hardened) or a hardening option (in the opposite case). For syst=
emd I see two approaches:
>=20
> 1. Default noexec /dev, override with something like
> - ExecPaths=3D/dev
> - MountOptions=3D/dev:rw,exec,dev,nosuid
> - or even MountOptions=3D/dev/sgx:rw,exec,dev,nosuid
> - ProtectDev=3Dno
> - AllowSGX=3Dyes
>=20
> 2. Default exec /dev, override with
> - NoExecPaths=3D/dev
> - MountOptions=3D/dev:rw,noexec,dev,nosuid
> - ProtectDev=3Dyes
> - DenySGX=3Dyes
>=20
> I'd prefer 1. but of course 2. would be reasonable.
>=20
>> To summarize, I neither understand the intended target audience.
>=20
> We have something in common: me neither. What's the target audience for=
 SGX? What's the use case? What are the users: browsers, system services?=
 How would applications use SGX? Should udev rules for /dev/sgx make it a=
vailable to any logged in users with uaccess tags?

Unless the system administrator/distribution has a specific reason to dis=
able SGX, /dev/sgx_enclave it should be generally available to any user p=
rocess (mode 0666). SGX just provides a different way for an application =
to execute code by using the ENCLU x86 instruction. Ultimately, any appli=
cation author could decide to use SGX in its design, including browsers, =
system services, network services, etc.

The device doesn't really provide any I/O or other hardware access. Even =
when the device is in use by a process, the OS has full control over sche=
duling and resource management. The only reason it's a device in /dev is =
to be able to provide an fd for use with mmap so that it can setup memory=
 ranges for use with the ENCLU instruction.

/dev/sgx_provision provides access to CPU hardware identifier and should =
be root/root 0600 to preserve privacy.

--
Jethro Beekman | Fortanix



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