On Sun, Feb 4, 2018 at 8:01 PM, Tycho Andersen <ty...@tycho.ws> wrote: > Hi Andy, > > On Sun, Feb 04, 2018 at 05:36:33PM +0000, Andy Lutomirski wrote: >> > The actual implementation of this is fairly small, although getting the >> > synchronization right was/is slightly complex. Also worth noting that there >> > is one race still present: >> > >> > 1. a task does a SECCOMP_RET_USER_NOTIF >> > 2. the userspace handler reads this notification >> > 3. the task dies >> > 4. a new task with the same pid starts >> > 5. this new task does a SECCOMP_RET_USER_NOTIF, gets the same cookie id >> > that the previous one did >> > 6. the userspace handler writes a response >> >> I'm slightly confused. I thought the id was never reused for a given >> struct seccomp_filter. (Also, shouldn't the id be u64, not u32?) > > Well, what happens when u32/64 overflows? Eventually it will wrap.
I think we can safely assume that u64 won't overflow. Even if we processed one user return notification on a given seccomp_filter every nanosecond (which would be insanely fast), that's 584 years. > >> On very quick reading, I have a question. What happens if a process >> has two seccomp_filters attached, one of them returns >> SECCOMP_RET_USER_NOTIF, and the *other* one has a listener? > > Good question, in seccomp_run_filters(), the first (lowest, last > applied) filter who returns SECCOMP_RET_USER_NOTIF is the one that > gets the notification and the other receives nothing. > > I don't really have any reason to prefer this behavior, it's just what > happened without much thought. Hmm. This won't nest right. Maybe we should just disallow a user-notification-using filter from being applied if there is already one in the stack. Then, if anyone cares about making these things nest right, they can fix it.