Comment 0 for bug 1830859

It started with some of my usual KVM checks and found them failing on Disco with:
  error: internal error: process exited while connecting to monitor: 2019-05-28T17:10:17.121934Z qemu-system-x86_64: -sandbox on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny: failed to install seccomp syscall filter in the kernel

I realized it works on X/B/C/E but not in a D container
It worked ~4 weeks ago.

This test can be simplified to one command:
$ qemu-system-x86_64 -enable-kvm -nographic -nodefaults -sandbox on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny

With strace I found different behavior:
Good:
[pid 3487] 0.000000 seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_TSYNC, NULL) = -1 EFAULT (Bad address) <0.000011>
[pid 3487] 0.003136 seccomp(SECCOMP_SET_MODE_STRICT, 1, NULL) = -1 EINVAL (Invalid argument) <0.000044>
[pid 3487] 0.000250 seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_TSYNC, {len=68, filter=0x55f2b7a5e2d0}) = 0 <0.000251>

Bad:
[pid 484] 0.000000 seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_TSYNC, NULL) = -1 EFAULT (Bad address) <0.000017>
[pid 484] 0.002680 seccomp(SECCOMP_GET_ACTION_AVAIL, 0, [SECCOMP_RET_KILL_PROCESS]) = 0 <0.000014>
[pid 484] 0.000088 seccomp(SECCOMP_SET_MODE_STRICT, 1, NULL) = -1 EINVAL (Invalid argument) <0.000013>

The difference seems to come from being built against seccomp as in Disco (2.3.3-3ubuntu2 = good) or Disco-proposed ( = bad).

The dependency on the built package stayed
 libseccomp2 (>= 2.3.0)
It did not pick up 2.4 via e.g. shlibs.

If I install libseccomp2 2.4 from -proposed the issue is gone.
Strace now looks like:
[pid 1788] 0.000000 seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_TSYNC, NULL) = -1 EFAULT (Bad address) <0.000015>
[pid 1788] 0.004167 seccomp(SECCOMP_GET_ACTION_AVAIL, 0, [SECCOMP_RET_KILL_PROCESS]) = 0 <0.000013>
[pid 1788] 0.000098 seccomp(SECCOMP_SET_MODE_STRICT, 1, NULL) = -1 EINVAL (Invalid argument) <0.000013>
[pid 1788] 0.000054 seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_TSYNC, NULL) = -1 EFAULT (Bad address) <0.000022>
[pid 1788] 0.000061 seccomp(SECCOMP_GET_ACTION_AVAIL, 0, [SECCOMP_RET_KILL_PROCESS]) = 0 <0.000017>
[pid 1788] 0.001477 seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_TSYNC, {len=68, filter=0x564de3852560}) = 0 <0.000288>

This is in all releases proposed to fix CVE-2019-9893
I was just ?lucky? to pick it up with my qemu build in the PPA that has proposed enabled.

Now there might be some toolchain detail to this as well.
As I built qemu for B/C as well and they built against the proposed 2.4 versions as well.
But in B/C things work with new qemu builds and old libseccomp2 installed.
Eoan on the other hand works by having libseccomp2 2.4.1-0ubuntu0.19.10.3 already migrated.

But that "luck" of B/C working might be specific to Qemus code, other rebuilds against the new libseccomp2 might fail as well in B/C and further. Shlibs detection is based on these symbols but my new qemu build is not calling these so it dependency stays at 2.3.

### some related Build logs ###
Cosmic rebuild against 2.4, not affected by the issue
https://launchpad.net/~ci-train-ppa-service/+archive/ubuntu/3729/+build/16858331/+files/buildlog_ubuntu-cosmic-amd64.qemu_1%3A2.12+dfsg-3ubuntu8.9~ppa1_BUILDING.txt.gz

Disco rebuild against 2.4, affected by the issue
https://launchpadlibrarian.net/425689546/buildlog_ubuntu-disco-amd64.qemu_1%3A3.1+dfsg-2ubuntu3.2~ppa1_BUILDING.txt.gz

Disco build against 2.3 (working)
https://launchpadlibrarian.net/422884897/buildlog_ubuntu-disco-amd64.qemu_1%3A3.1+dfsg-2ubuntu3.1_BUILDING.txt.gz