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Re: [Qemu-devel] [PATCH for-1.7] seccomp: setting "-sandbox on" by defau

From: Eduardo Otubo
Subject: Re: [Qemu-devel] [PATCH for-1.7] seccomp: setting "-sandbox on" by default
Date: Thu, 21 Nov 2013 14:22:32 -0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130912 Thunderbird/17.0.9

On 11/21/2013 01:48 PM, Paul Moore wrote:
On Thursday, November 21, 2013 04:14:11 PM Paolo Bonzini wrote:
Il 30/10/2013 11:04, Stefan Hajnoczi ha scritto:
On Wed, Oct 23, 2013 at 12:42:34PM -0200, Eduardo Otubo wrote:
On 10/22/2013 11:00 AM, Anthony Liguori wrote:
On Tue, Oct 22, 2013 at 12:21 PM, Eduardo Otubo

<address@hidden> wrote:
Inverting the way sandbox handles arguments, making possible to have no
argument and still have '-sandbox on' enabled.

Signed-off-by: Eduardo Otubo <address@hidden>

The option '-sandbox on' is now used by default by virt-test[0] -- it
has been merged into the 'next' branch and will be available in the
next release, meaning we have a back support for regression tests if
anything breaks because of some missing system call not listed in the

This being said, I think it makes sense to have this option set to 'on'
default in the next Qemu version. It's been a while since no missing
syscall is reported and at this point the whitelist seems to be pretty

[0] -
This breaks hot_add of a network device that uses a script= argument,

If so, this cannot be made default.

Anthony, I believe you're talking about the blacklist feature. This
is the old whitelist that is already upstream and it does not block
any network device to be hot plugged.

The following fails to start here (the shell hangs and ps shows QEMU is
a <defunct> process):

qemu-system-x86_64 -sandbox on -enable-kvm -m 1024 -cpu host \

                    -drive if=virtio,cache=none,file=test.img

Easier-to-debug failures are another prerequisite for enabling the
sandbox by default, I think.

I've already sent a patch for a "debug mode" in the past. It was denied because of two main points: (1) Anthony was looking for a more solid and closed solution for sandboxing. A "debug" or "learning" mode would be too much exposure of the attack surface. (2) Debug mode was changing the Qemu's sig mask in a way that it was breaking it. And besides, at that time, there were too many linked libraries that would interfere on this sig handling, so we gave up on this feature.

I believe I've posted this information before, but just in case ...

IMHO, it is really not that hard to debug a seccomp failure; the first step is
to look for the failure in the audit log or syslog.  If you are on a
Fedora/RHEL based system you are most likely running audit, so finding the
seccomp failures are quite simple with the 'ausearch' command:

  # ausearch -m SECCOMP
  time->Wed Nov 20 09:52:08 2013
  type=SECCOMP msg=audit(1384912328.482:6656): auid=0 uid=0 gid=0 ses=854
   subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 pid=12087
   comm="qemu-kvm" sig=31 syscall=62 compat=0 ip=0x7f7a1d2abc67 code=0x0

... if you are using syslog, feel free to use whatever tool you prefer, e.g.
grep, less, etc.

Once you have the syscall number, "syscall=62", in the audit message above,
you can use the 'scmp_sys_resolver' to resolve the number into a name:

  # scmp_sys_resolver 62

The 'scmp_sys_resolver' tool is part of the libseccomp-devel package on
Fedora/RHEL based systems, it may be packaged differently on other

I'm always open to suggestions on how to improve the development/debugging
process, so if you have any ideas please let me know.

Also, I've been working on some improvements on virt-test side. I'm trying to make it report the illegal system call using audit log and libseccomp as source of information. This way a simple run could identify missing system calls for the whitelist.

Eduardo Otubo
IBM Linux Technology Center

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