Re: BTI interaction between seccomp filters in systemd and glibc mprotect calls, causing service failures

Topi Miettinen
2020-10-24T11:34:33Z

On 23.10.2020 20.52, Salvatore Mesoraca wrote:
> Hi,
>
> On Thu, 22 Oct 2020 at 23:24, Topi Miettinen <toiwoton@gmail.com> wrote:
>> SARA looks interesting. What is missing is a prctl() to enable all W^X
>> protections irrevocably for the current process, then systemd could
>> enable it for services with MemoryDenyWriteExecute=yes.
>
> SARA actually has a procattr[0] interface to do just that.
> There is also a library[1] to help using it.

That means that /proc has to be available and writable at that point, so
setting up procattrs has to be done before mount namespaces are set up.
In general, it would be nice for sandboxing facilities in kernel if
there would be a way to start enforcing restrictions only at next
execve(), like setexeccon() for SELinux and aa_change_onexec() for
AppArmor. Otherwise the exact order of setting up various sandboxing
options can be very tricky to arrange correctly, since each option may
have a subtle effect to the sandboxing features enabled later. In case
of SARA, the operations done between shuffling the mount namespace and
before execve() shouldn't be affected so it isn't important. Even if it
did (a new sandboxing feature in the future would need trampolines or
JIT code generation), maybe the procattr file could be opened early but
it could be written closer to execve().

-Topi