Re: [RFC 0/3] seccomp trap to userspace

[Christian Brauner <christian.brauner@mailbox.org>]

On Fri, Mar 16, 2018 at 09:01:47AM -0700, Andy Lutomirski wrote:
> 
> 
> > On Mar 16, 2018, at 7:47 AM, Christian Brauner <christian.brauner@mailbox.org> wrote:
> > 
> >> On Fri, Mar 16, 2018 at 12:46:55AM +0000, Andy Lutomirski wrote:
> 
> 
> I bet I confused everyone with a blatant typo:
> 
> >> 
> >> Hmm, I think we have to be very careful to avoid nasty races.  I think
> >> the correct approach is to notice the signal and send a message to the
> >> listener that a signal is pending but to take no additional action.
> >> If the handler ends up completing the syscall with a successful
> >> return, we don't want to replace it with -EINTR.  IOW the code looks
> >> kind of like:
> >> 
> >> send_to_listener("hey I got a signal");
> 
> That should be “hey I got a syscall”.   D’oh!

Ha ok, that's what led me to believe that listener != handler and I was
trying to make sense of thise. :)

Thanks!
Christian

> 
> >> wait_ret = wait_interruptible for the listener to reply;
> >> if (wait_ret == -EINTR) {
> > 
> > Hm, so from the pseudo-code it looks like: The handler would inform the
> > listener that it received a signal (either from the syscall requester or
> > from somewhere else) and then wait for the listener to reply to that
> > message.  This would allow the listener to decide what action it wants
> > the handler to take based on the signal, i.e. either cancel the request
> > or retry?  The comment makes it sound like that the handler doesn't
> > really wait on the listener when it receives a signal it simply moves
> > on.
> 
> It keeps waiting killably but not interruptibly. 
> 
> > So no "taking no additional action" here means not have the handler
> > decide to abort but the listener?
> 
> If by “handler” you mean kernel, then yes. 
> 
> There’s no userspace syscall handler involved. From the kernel’s perspective, a syscall is never still in progress when a signal handler is invoked — we only actually invoke syscall handlers in prepare_exit_to_usermode() or the non-x86 equivalent and the functions it calls. While a syscall is running, the kernel might notice that a signal is pending and do one of a few things:
> 
> 1. Just keep going. Not all syscalls can be interrupted. 
> 
> 2. Try to finish early. If a send() call has already sent some but not all data, it can stop waiting and return the number of bytes sent.
> 
> 3. Abort with -EINTR.
> 
> 4. Abort with -ERESTARTSYS or one of its relatives. These fiddle with user registers in a somewhat unpleasant way to pretend that the syscall never actually happened.  This works for syscalls that wait with an absolute timeout, for example. 
> 
> 5. Set up restart_syscall() magic, rewrite regs so it looks like the user was about to call restart_syscall() when the signal happened, and abort. 
> 
> In all cases, the signal is dealt with afterwards. This could result in changing regs to call the handler or in simply returning. 
> 
> 1-3 should work fully in seccomp. The only issue is that the kernel doesn’t know *which* to do, nor can the kernel force the listener to abort cleanly, so I think we have  no real choice but to let the listener decide. 
> 
> 4 could be supported just like 1-3. 5 is awful, and I don’t think we should support it for user listeners.