Re: [Qemu-devel] [PATCH] Add syscalls for -runas and -chroot tothe seccomp sandbox

[Markus Armbruster <armbru@redhat.com>]

"Namsun Ch'o" <namnamc@Safe-mail.net> writes:

>> Drawback: complexity.  If we decide to limit ourselves to the original
>> threat model (rogue guest), and enter the sandbox only after setup, we
>> can keep things simpler.
>
> We could do both without much complexity. This looks simple enough to me:
>
>   rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(chroot), 1,
>         SCMP_A0(SCMP_CMP_EQ, chroot_dir));
>   if (rc < 0)
>         goto seccomp_return;
>
>   rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(chdir), 1,
>         SCMP_A0(SCMP_CMP_EQ, "/"));
>   if (rc < 0)
>         goto seccomp_return;
>
> The only time chroot_dir is ever used is in os-posix.c:139:
>
>   if (chroot(chroot_dir) < 0) {

I'm afraid this materially weakens the sandbox.  chroot_dir is writable.

We don't need to permit chroot(chroot_dir) if we enter the sandbox only
after setup.