On Tue, Jan 20, 2015 at 3:04 PM, Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> wrote: > On Tue, Jan 20, 2015 at 10:45:19PM +0000, Russell King - ARM Linux wrote: >> Well, the whole question is this: is restarting a system call like >> usleep() really a separate system call, or is it a kernel implementation >> detail? >> >> If you wanted seccomp to see this, what would be the use case? Why >> would seccomp want to block this syscall? Does it make sense for >> seccomp to block this syscall when it doesn't block something like >> usleep() and then have usleep() fail just because the thread received >> a signal? >> >> I personally regard the whole restart system call thing as a purely >> kernel internal thing which should not be exposed to userland. If >> we decide that it should be exposed to userland, then it becomes part >> of the user ABI, and it /could/ become difficult if we needed to >> change it in future - and I'd rather not get into the "oh shit, we >> can't change it because that would break app X" crap. > > Here's a scenario where it could become a problem: > > Let's say that we want to use seccomp to secure some code which issues > system calls. We determine that the app uses system calls which don't > result in the restart system call being issued, so we decide to ask > seccomp to block the restart system call. Some of these system calls > that the app was using are restartable system calls. > > When these system calls are restarted, what we see via ptrace etc is > that the system call simply gets re-issued as its own system call. > > In a future kernel version, we decide that we could really do with one > of those system calls using the restart block feature, so we arrange > for it to set up the restart block, and return -ERESTART_BLOCK. That's > fine for most applications, but this app now breaks. > > The side effect of that breakage is that we have to revert that kernel > change - because we've broken userland, and that's simply not allowed. > > Now look at the alternative: we don't make the restart syscall visible. > This means that we hide that detail, and we actually reflect the > behaviour that we've had for the other system call restart mechanisms, > and we don't have to fear userspace breakage as a result of switching > from one restart mechanism to another. > > I am very much of the opinion that we should be trying to limit the > exposure of inappropriate kernel internal details to userland, because > userland has a habbit of becoming reliant on them, and when it does, > it makes kernel maintanence unnecessarily harder. I totally agree with you. :) My question here is more about what we should do with what we currently have since we have some unexpected combinations. There is already an __NR_restart_syscall syscall and it seems like it's already part of the userspace ABI: - it is possible to call it from userspace directly - seccomp "sees" it - ptrace doesn't see it Native ARM64 hides the restart from both seccomp and ptrace, and this seems like the right idea, except that restart_syscall is still callable from userspace. I don't think there's a way to make that vanish, which means we'll always have an exposed syscall. If anything goes wrong with it (which we've been quite close to recently[1]), there would be no way to have seccomp filter it. So, at the least, I'd like arm64 to NOT hide restart_syscall from seccomp, and at best I'd like both arm and arm64 to (somehow) entirely remove restart_syscall from the userspace ABI so it wouldn't need to be filtered, and wouldn't become a weird ABI hiccup as you've described. I fail to imagine a way to remove restart_syscall from userspace, so I'm left with wanting parity of behavior between ARM and ARM64 (and x86). What's the right way forward? -Kees [1] https://git.kernel.org/linus/849151dd5481bc8acb1d287a299b5d6a4ca9f1c3 -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html