On 15.02.2017 19:27, 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.
I like your proposal! I just wanted to add an idea for an additional parameter (not sure whether it is feasible, though): Something like "-sandbox on,network=off" ... i.e. forbid all system calls that are used for networking. Rationale: Sometimes your VM does not need any networking, and you want to make sure that a malicious guest can also not reach your local network in that case. Thomas