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  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?
>> Typo: "of" = "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 anywhere
>> 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 about
>> *default installation* of a distribution. That should have somewhat sane
>> 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.
> I think we agree that there's a need for either way to allow SGX (if default is hardened) or a hardening option (in the opposite case). For systemd I see two approaches:
> 1. Default noexec /dev, override with something like
> - ExecPaths=/dev
> - MountOptions=/dev:rw,exec,dev,nosuid
> - or even MountOptions=/dev/sgx:rw,exec,dev,nosuid
> - ProtectDev=no
> - AllowSGX=yes
> 2. Default exec /dev, override with
> - NoExecPaths=/dev
> - MountOptions=/dev:rw,noexec,dev,nosuid
> - ProtectDev=yes
> - DenySGX=yes
> I'd prefer 1. but of course 2. would be reasonable.
>> To summarize, I neither understand the intended target audience.
> 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 available to any logged in users with uaccess tags?

Unless the system administrator/distribution has a specific reason to disable SGX, /dev/sgx_enclave it should be generally available to any user process (mode 0666). SGX just provides a different way for an application to execute code by using the ENCLU x86 instruction. Ultimately, any application 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 scheduling 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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