On Thu, Feb 16, 2017 at 09=33=16AM +0000, Daniel P. Berrange wrote: > 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:
[...] > > > > > > 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)'). I've been working on the blacklist, you can see here: https://github.com/otubo/qemu/commit/31e603180081474ff35c5897813cb635f8e9a786 I didn't send as an RFC to the list because it's still an on going work, but if you have any comments, please feel free. > > > > > > 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. The obsolete part is also on my github (didn't send for the same reason): https://github.com/otubo/qemu/commit/54a57eb150ca3e5b67e9a81394c6cfa4ac82a6ff Also, can't find anywhere a solid list of obsolete system calls, can you elaborate a little more on how to determine this list? > > > > > > 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. Also, I was re-reading all documentation again, prctl(PR_SET_NO_NEW_PRIVS) is enabled by default when using seccomp. > > > > > > 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/ :| > -- Eduardo Otubo ProfitBricks GmbH
