SECCOMP_IOCTL_NOTIF_ADDFD race condition

[Sargun Dhillon]

On Tue, Dec 01, 2020 at 07:41:05AM -0500, Tycho Andersen wrote:
> On Mon, Nov 30, 2020 at 06:20:09PM -0500, Tycho Andersen wrote:
> > Idea 1 sounds best to me, but maybe that's because it's the way I
> > originally did the fd support that never landed :)
> > 
> > But here's an Idea 4: we add a way to remotely close an fd (I don't
> > see that the current infra can do this, but perhaps I didn't look hard
> > enough), and then when you get ENOENT you have to close the fd. Of
> > course, this can't be via seccomp, so maybe it's even more racy.
> 
> Or better yet: what if the kernel closed everything it had added via
> ADDFD if it didn't get a valid response from the supervisor? Then
> everyone gets this bug fixed for free.
> 
> Tycho
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This doesn't solve the problem universally because of the (Go) preemption 
problem. Unless we can guarantee that the supervisor can always handle the 
request in fewer than 10ms, or if it implements resumption behaviour. I know 
that resumption behaviour is a requirement no matter what, but the easier we can 
make it to implement resumption, the better chance we are giving users to get 
this right.

I think that the easiest solution is to add to the SECCOMP_IOCTL_NOTIF_RECV
ioctl. Either we have a flag like "block all blockable signals" or pass a
sigset_t directly of which signals to allow (and return an einval if they
try to block non-blockable signals).