On Thu, Feb 16, 2017 at 12:36:51AM +0100, Eduardo Otubo wrote: > On Wed, Feb 15, 2017 at 06=27=32PM +0000, Daniel P. Berrange wrote: > > The current impl of seccomp in QEMU is intentionally allowing a huge range > > of system calls to be executed. The goal was that running '-sandbox on' > > should never break any feature of QEMU, so naturally any syscall that can > > executed on any codepath QEMU takes must be allowed. > > > > This is good for usability because users don't need to understand the > > technical > > details of the sandbox technology, they merely say "on" and it "just works". > > Conversely though, this is bad for security because QEMU has to allow a huge > > range of system calls to be used due to its broad functionality. > > > > During initial discussions for seccomp back in 2012 it was suggested, there > > might be alternate policies developed for QEMU which deny some features, but > > improve security overall. To best of my knowledge, this has never been > > discussed > > again since then. > > > > > > In addition, since initially merging, there has been a steady stream of > > patches > > to whitelist further syscalls that were missing. Some of these were missing > > due > > to newly added functionality in QEMU since the original seccomp impl, while > > others have been missing since day 1. It is reasonable to expect that there > > are > > still many syscalls missing in the whitelist. In just a couple of minutes of > > comparing the whitelist vs global syscall list it was possible to identify > > two > > further missing syscalls. The '-netdev bridge,br=virbr0' network backend > > fails > > because setuid is blocked, preventing execution of the qemu-bridge-helper > > program. If built against glibc < 2.9, or running on kernel < 2.6.27 it will > > fail to call eventfd() because we only permit eventfd2() syscall, not the > > older eventfd() syscall used on older Linux. Some ifup scripts used with the > > -netdev arg may also break due to lack of chmod, flock, getxattr > > permissions. > > This risk of missing syscalls is why -sandbox defaults to off, and we've > > never > > considered defaulting it to on. > > > > > > The fundamental problem is that building a whitelist of syscalls used by > > QEMU > > emulators is an intractable problem. QEMU on my system links to 183 > > different > > shared libraries and there is no way in the world that anyone can figure out > > which code paths QEMU triggers in these libraries and thus identify which > > syscalls will be genuinely needed. > > > > Thus a whitelist based approach for QEMU is doomed to always be missing some > > syscalls, resulting in uneccessary abrts of QEMU when it tickles some edge > > case. If you are lucky the abort() happens at startup so you see it quickly > > and can address it. If you are unlucky the abort() happens after your VM has > > been running for days/week/months and you loose data. > > > > IOW, seccomp integration as it currently exists today in QEMU offers minimal > > security benefits, while at the same time causing spurious crashes which may > > cause user data loss from aborting a running VM, discouraging users from > > using > > even the minimal protection it offers. > > > > I think we need to rework our seccomp support so that we can have a high > > enough > > level of confidence in it, that it could be enabled by default. At the same > > time > > we need to make it do something more tangibly useful from a security POV. > > > > > > First we need to admit that whitelisting is a failed approach, and switch to > > using blacklisting. Unless we do this, we'll never have high enough > > confidence > > to enable it by default - something that's never turned on might as well not > > exist at all. > > > > > > There is a reasonable easily identifiable set of syscalls that QEMU should > > never be permitted to use, no matter what configuration it is in, what > > helpers > > it spawns, or what libraries it links to. eg reboot, swapon, swapoff, > > syslog, > > mount, unmount, kexec_*, etc - any syscall that affects global system state, > > rather than process local state should be forbidden. > > > > There are some syscalls that are simply hardcoded to return ENOSYS which can > > be trivially blacklisted. afs_syscall, break, fattach, ftime, etc (see the > > man page 'unimplemented(2)'). > > > > There are some syscalls which are considered obsolete - they were previously > > useful, but no modern code would call them, as they have been superceeded. > > For example, readdir replaced by getdents. We could blacklist these by > > default > > but provide a way to allow use of obsolete syscalls if running on older > > systems. > > e.g. '-sandbox on,obsolete=allow'. They might be obsolete enough that we > > decide > > to just block them permanently with no opt in - would need to analyse when > > their replacements appeared in widespread use. > > > > There might be a few more syscalls which we can determine are never valid to > > use in QEMU or any library or helper program it might run. I expect this > > list > > to be very small though, given the impossibility of auditing code paths > > through > > millions of lines of code QEMU links to. > > > > Everything else should be allowed. > > > > At this point we have a highly reliable "-sandbox on" which we're not having > > to constantly patch. > > > > > > From here we need a way to allow a user to opt-in to more restrictive > > policies, > > accepting that it will block certain features. For example, there should be > > a > > a way to disable any means to elevate privileges from QEMU or things it > > spawns. > > e.g. '-sandbox on,elevateprivileges=deny'. > > > > This would not only block the variuous set*uid|gid functions via seccomp, > > but > > should also prctl(PR_SET_NO_NEW_PRIVS). This would allows the user to optin > > to > > a restrictive world if they know they'll not require things like the setuid > > bridge helper. > > > > Similarly there should be an '-sandbox on,spawn=deny' which prevents the > > ability > > to fork/exec processes at all, whether privileged or not. This would block > > features like the qemu bridge helper, SMB server, ifup/down scripts, > > migration > > exec: protocol. These are all rarely used features though, so an opt-in to > > block > > their use is reasonable & desirable. > > > > A -sandbox on,resourcecontrol=deny, which prevents QEMU from setting stuff > > like > > process affinity, schedular priority, etc. Some uses of QEMU might need > > them, > > but normally such controls are left to the mgmt app above QEMU to set prior > > to > > the exec() of QEMU. > > > > > > > > The key is that these are *not* low level knobs controlling system calls, > > but > > moderately high level knobs controlling general concepts. This is a high > > enough > > level of abstraction to enable libvirt to automatically turn them on/off > > based > > on guest config, without libvirt having to know anything detailed about QEMU > > code impl for the features. > > > > > > Finally, for avoidance of doubt, I'm *not* actually proposing to implement > > this > > myself any time in the forseeable future. This mail came about from the fact > > that many people have questioned whether current seccomp code is anything > > other > > than "security theatre". I tend to agree with such an assessment myself, > > and was > > initially intending to just send a patch to remove seccomp, to stimulate > > some > > discussion. Instead, however, I decided to write this mail to see if we can > > identify a way forward to make seccomp both reliable and useful. If QEMU > > had the > > kind of approach outlined above, with a default blacklist instead of > > whitelist, > > and some opt-ins for stricter lists, it is something I think libvirt would > > be > > reasonably happy to enable out of the box. That would be a step forward from > > today where libvirt would never consider turning seccomp on by default. > > > > Perhaps this re-working could be a GSoC idea for some interested student... > > > > I'm not a student, thus not eligible GSoC person but I would be more > than grateful to take this initiative of yours and transform into some > patches so we can make this feature something really useful and > reliable.
Sure, I just threw GSoC out there as one possible idea. If you or anyone else has time to work on it, that's great too. > Perhaps now is not the right time to terse comments on every idea you > gave, I agree with most of them. I wrote the whole implementation of > this feature but actually became the maintainer because people approving > sycalls and sending pull-requests were too busy, and I could do it. But > to be completely honest I had few poor ideas on how to improve it and > almost no time to actually do it in the past. Time passed by and all I > did was approve new syscalls and turn them into pull-requests. > > Let's spin up these ideas and hopefully incorporate into Qemu. Next step > I'm gonna dig into every topic and draft a little more. I guess we can > keep on this thread, or perhaps in separate ones. From there I can start > to write some code. ok Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://entangle-photo.org -o- http://search.cpan.org/~danberr/ :|