On Thu, Mar 29, 2018 at 09:54:01AM -0400, Mathieu Desnoyers wrote: > Let's say we disallow system calls from rseq critical sections. A few points > arise: > > - We still need to allow traps (page faults, breakpoints, ...) within rseq c.s., > > - We still need to allow interrupts within rseq c.s., Sure, but all those are different entry points, so that shouldn't be a problem. > - We need to decide whether we just document that syscalls within rseq c.s. > are not supported, or we enforce a behavior if this happens (e.g. SIGSEGV). > If we enforce a SIGSEGV, we'd have to figure out whether it's worth it to > add extra branches to the system call fast path to validate this. Without enforcement someone will eventually do this :/ We might (maybe) get away with it being a debug option somewhere, but even that sounds like trouble. > - We need to carefully consider the case of system calls issued within signal > handlers nested on top of rseq. When RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL is > _not_ set, neither in the rseq c.s. descriptor nor in the TLS @flags, > it's pretty much straightforward: upon signal delivery, the kernel moves the > ip to abort, and clears the tls @rseq_cs pointer. This means that any system > call issued within the signal handler is not actually within the rseq c.s. > upon which the signal is nested. > > The case I worry about is if a thread sets the RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL > flag in its TLS @flags field (useful in a debugging scenario where we want a > debugger to single-step through the rseq c.s. and observe registers at each step). > Arguably, this is only ever used in development. However, it does allow a situation > where a system call executed within a signal handler can nest over a rseq c.s.. > So if we choose to be very strict and SIGSEGV any syscall nested over rseq > c.s., we may very well end up killing the process for no good reason in this > scenario. Yes, that needs a little thought; but when we run the signal handler, the IP would no longer be inside the active RSEQ, right? > - We need to decide whether all syscalls are disallowed, or if we want to pick > specific ones (e.g. fork()). All. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
