[systemd-devel] Creating executable device nodes in /dev?

[Topi Miettinen]

On 19.11.2020 18.32, Zbigniew Jędrzejewski-Szmek wrote:
> On Thu, Nov 19, 2020 at 08:17:08AM -0800, Andy Lutomirski wrote:
>> Hi udev people-
>>
>> The upcoming Linux SGX driver has a device node /dev/sgx.  User code
>> opens it, does various setup things, mmaps it, and needs to be able to
>> create PROT_EXEC mappings.  This gets quite awkward if /dev is mounted
>> noexec.
>>
>> Can udev arrange to make a device node executable on distros that make
>> /dev noexec?  This could be done by bind-mounting from an exec tmpfs.
>> Alternatively, the kernel could probably learn to ignore noexec on
>> /dev/sgx, but that seems a little bit evil.
> 
> I'd be inclined to simply drop noexec from /dev by default.
> We don't do noexec on either /tmp or /dev/shm (because that causes immediate
> problems with stuff like Java and cffi). And if you have those two at your
> disposal anyway, having noexec on /dev doesn't seem important.

I'd propose to not enable exec globally, but if a service needs SGX, it 
could use something like MountOptions=/dev:exec only in those cases 
where it's needed. That way it's possible to disallow writable and 
executable file systems for most services (which typically don't need 
/tmp or /dev/shm either). Of course the opposite 
(MountOptions=/dev:noexec) would be also possible, but I'd expect that 
this would be needed to be used more often.

-Topi