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Re: [Qemu-devel] RFC: How to make seccomp reliable and useful ?

From: Thomas Huth
Subject: Re: [Qemu-devel] RFC: How to make seccomp reliable and useful ?
Date: Thu, 16 Feb 2017 09:38:59 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0

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.


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