Re: [PATCH v4] seccomp: add ptrace options for suspend/resume

Quoting Andy Lutomirski (luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org):

On Wed, Jun 10, 2015 at 9:31 AM, Oleg Nesterov [off-list ref] wrote:

quoted

On 06/09, Andy Lutomirski wrote:

quoted

On Tue, Jun 9, 2015 at 5:49 PM, Tycho Andersen

quoted

@@ -556,6 +556,15 @@ static int ptrace_setoptions(struct task_struct *child, unsigned long data)
        if (data & ~(unsigned long)PTRACE_O_MASK)
                return -EINVAL;

+       if (unlikely(data & PTRACE_O_SUSPEND_SECCOMP)) {

Well, we should do this if

                        (data & O_SUSPEND) && !(flags & O_SUSPEND)

or at least if

                        (data ^ flags) & O_SUSPEND

quoted

quoted

+               if (!config_enabled(CONFIG_CHECKPOINT_RESTORE) ||
+                   !config_enabled(CONFIG_SECCOMP))
+                       return -EINVAL;
+
+               if (!capable(CAP_SYS_ADMIN))
+                       return -EPERM;

I tend to think that we should also require that current not be using
seccomp.  Otherwise, in principle, there's a seccomp bypass for
privileged-but-seccomped programs.

Andy, I simply can't understand why do we need any security check at all.

OK, yes, in theory we can have a seccomped CAP_SYS_ADMIN process, seccomp
doesn't filter ptrace, you hack that process and force it to attach to
another CAP_SYS_ADMIN/seccomped process, etc, etc... Looks too paranoid
to me.

I've sometimes considered having privileged processes I write fork and
seccomp their child.  Of course, if you're allowing ptrace through
your seccomp filter, you open a giant can of worms, but I think we
should take the more paranoid approach to start and relax it later as

I really do intend to look at your old proposed tree for improving that...
have only done a once-over so far, though.

needed.  After all, for the intended use of this patch, stuff will
break regardless of what we do if the ptracer is itself seccomped.

I could be convinced that if the ptracer is outside seccomp then we
shouldn't need the CAP_SYS_ADMIN check.  That would at least make this
work in a user namespace.

quoted

quoted

quoted

@@ -590,6 +590,10 @@ void secure_computing_strict(int this_syscall)
 {
        int mode = current->seccomp.mode;

+       if (config_enabled(CONFIG_CHECKPOINT_RESTORE) &&
+           unlikely(current->ptrace & PT_SUSPEND_SECCOMP))
+               return;
+
        if (mode == 0)
                return;
        else if (mode == SECCOMP_MODE_STRICT)

@@ -691,6 +695,10 @@ u32 seccomp_phase1(struct seccomp_data *sd)
        int this_syscall = sd ? sd->nr :
                syscall_get_nr(current, task_pt_regs(current));

+       if (config_enabled(CONFIG_CHECKPOINT_RESTORE) &&
+           unlikely(current->ptrace & PT_SUSPEND_SECCOMP))
+               return SECCOMP_PHASE1_OK;
+

If it's not hard, it might still be nice to try to fold this into
mode.  This code is rather hot.  If it would be a mess, then don't
worry about it for now.

IMO, this would be a mess ;) At least compared to this simple patch.

Suppose we add SECCOMP_MODE_SUSPENDED. Not only this adds the problems
with detach if the tracer dies.

We need to change copy_seccomp(). And it is not clear what should we
do if the child is traced too.

We need to change prctl_set_seccomp() paths.

And even the "tracee->seccomp.mode = SECCOMP_MODE_SUSPENDED" code needs
some locking even if the tracee is stopped, we need to avoid the races
with SECCOMP_FILTER_FLAG_TSYNC from other threads.

Agreed.  Let's hold off until this becomes a problem (if it ever does).

--Andy