Is it safe to modify sandbox-seccomp-filter?

[Damien Miller]

On Mon, 18 Jun 2018, Wenyi Cheng wrote:

> Hi openssh-unix-dev,
> 
> I'm upgrading the openssh in our system from 6.6 to 7.6. The option
> UsePrivilegeSeparation for sshd has been deprecated since 7.5. We used to
> set it to yes but it's now sandbox by default.
> 
> We are using futex which is not allowed with sandbox. So I have to manually
> add the following code change to sandbox-seccomp-filter.c to make it work.
> 
> > #ifdef __NR_futex
> >         SC_ALLOW(__NR_futex),
> > #endif
> 
> 
> Will this change cause any security issue?

For the futex(2) syscall, probably not. 

Barring a kernel bug in the futex implementation itself, I don't think
it likely that it would create a new vulnerability. It could be useful
to an attacker who already has a vulnerability (esp a race condition)
and is struggling to reliably exploit it though.

-d