Re: [Qemu-devel] [RFC] [PATCHv2 2/2] Adding basic calls to libseccomp in vl.c

[Corey Bryant]

On 06/18/2012 04:33 AM, Daniel P. Berrange wrote:
> On Fri, Jun 15, 2012 at 07:04:45PM +0000, Blue Swirl wrote:
> > On Wed, Jun 13, 2012 at 8:33 PM, Daniel P. Berrange <address@hidden> wrote:
> > > On Wed, Jun 13, 2012 at 07:56:06PM +0000, Blue Swirl wrote:
> > > > On Wed, Jun 13, 2012 at 7:20 PM, Eduardo Otubo <address@hidden> wrote:
> > > > > I added a syscall struct using priority levels as described in the
> > > > > libseccomp man page. The priority numbers are based to the frequency
> > > > > they appear in a sample strace from a regular qemu guest run under
> > > > > libvirt.
> > > > >
> > > > > Libseccomp generates linear BPF code to filter system calls, those rules
> > > > > are read one after another. The priority system places the most common
> > > > > rules first in order to reduce the overhead when processing them.
> > > > >
> > > > > Also, since this is just a first RFC, the whitelist is a little raw. We
> > > > > might need your help to improve, test and fine tune the set of system
> > > > > calls.
> > > > >
> > > > > v2: Fixed some style issues
> > > > >         Removed code from vl.c and created qemu-seccomp.[ch]
> > > > >         Now using ARRAY_SIZE macro
> > > > >         Added more syscalls without priority/frequency set yet
> > > > >
> > > > > Signed-off-by: Eduardo Otubo <address@hidden>
> > > > > ---
> > > > >   qemu-seccomp.c |   73 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> > > > >   qemu-seccomp.h |    9 +++++++
> > > > >   vl.c           |    7 ++++++
> > > > >   3 files changed, 89 insertions(+)
> > > > >   create mode 100644 qemu-seccomp.c
> > > > >   create mode 100644 qemu-seccomp.h
> > > > >
> > > > > diff --git a/qemu-seccomp.c b/qemu-seccomp.c
> > > > > new file mode 100644
> > > > > index 0000000..048b7ba
> > > > > --- /dev/null
> > > > > +++ b/qemu-seccomp.c
> > > > > @@ -0,0 +1,73 @@
> > > >
> > > > Copyright and license info missing.
> > > >
> > > > > +#include <stdio.h>
> > > > > +#include <seccomp.h>
> > > > > +#include "qemu-seccomp.h"
> > > > > +
> > > > > +static struct QemuSeccompSyscall seccomp_whitelist[] = {
> > > >
> > > > 'const'
> > > >
> > > > > +    { SCMP_SYS(timer_settime), 255 },
> > > > > +    { SCMP_SYS(timer_gettime), 254 },
> > > > > +    { SCMP_SYS(futex), 253 },
> > > > > +    { SCMP_SYS(select), 252 },
> > > > > +    { SCMP_SYS(recvfrom), 251 },
> > > > > +    { SCMP_SYS(sendto), 250 },
> > > > > +    { SCMP_SYS(read), 249 },
> > > > > +    { SCMP_SYS(brk), 248 },
> > > > > +    { SCMP_SYS(clone), 247 },
> > > > > +    { SCMP_SYS(mmap), 247 },
> > > > > +    { SCMP_SYS(mprotect), 246 },
> > > > > +    { SCMP_SYS(ioctl), 245 },
> > > > > +    { SCMP_SYS(recvmsg), 245 },
> > > > > +    { SCMP_SYS(sendmsg), 245 },
> > > > > +    { SCMP_SYS(accept), 245 },
> > > > > +    { SCMP_SYS(connect), 245 },
> > > > > +    { SCMP_SYS(bind), 245 },
> > > >
> > > > It would be nice to avoid connect() and bind(). Perhaps seccomp init
> > > > should be postponed to after all sockets have been created?
> > >
> > > If you want to migrate your guest, you need to be able to
> > > call connect() at an arbitrary point in the QEMU process'
> > > lifecycle. So you can't avoid allowing connect(). Similarly
> > > if you want to allow hotplug of NICs (and their backends)
> > > then you need to have both bind() + connect() available.
> >
> > That's bad. Migration could conceivably be extended to use file
> > descriptor passing, but hotplug is more tricky.
>
> As with execve(), i'm reporting this on the basis that on the previous
> patch posting I was told we must whitelist any syscalls QEMU can
> conceivably use to avoid any loss in functionality.

Thanks for pointing out syscalls needed for the whitelist.

As Paul has already mentioned, it was recommended that we restrict all 
of QEMU (as a single process) from the start of execution.  This is 
opposed to other options of restricting QEMU from the time that vCPUS 
start, further restricting based on syscall parms, or decomposing QEMU 
into multiple processes that are individually restricted with their own 
seccomp whitelists.

I think this approach is a good starting point that can be further tuned 
in the future.  And as with most security measures, defense in depth 
improves the cause (e.g. combining seccomp with DAC or MAC).

--
Regards,
Corey