** Changed in: linux (Ubuntu Xenial)
Status: In Progress => Fix Committed
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https://bugs.launchpad.net/bugs/1721676
Title:
implement errno action logging in seccomp for strict mode with snaps
Status in Snappy:
In Progress
Status in linux package in Ubuntu:
Fix Released
Status in linux source package in Xenial:
Fix Committed
Status in linux source package in Zesty:
Fix Committed
Status in linux source package in Artful:
Fix Released
Bug description:
A requirement for snappy is that security sandbox violations against
policy are logged. In this manner learning tools can be written to
parse the logs, etc and make developing on snappy easier.
The current default seccomp action, in strict mode. is to kill the
snap's thread that violated the policy but this is unfriendly to the
developer and to the user. The desired action is to block the illegal
system call and return an error with errno set to EPERM. However,
seccomp does not emit log events when it takes that action. Seccomp
should be updated to emit log events when taking the SECCOMP_RET_ERRNO
action and then snappy can switch to the using that action when
blocking illegal system calls.
[Impact]
Snapd needs a way to log SECCOMP_RET_ERRNO seccomp actions in order to
have a more friendly strict mode. Such functionality has been merged
upstream into 4.14-rc2.
No libseccomp changes are needed at this time since snap-confine loads
the BPF filter directly into the kernel without using libseccomp.
[Test Case]
Running the libseccomp "live" tests will exercise the kernel's seccomp
enforcement and help to help catch any regressions. Note that on
Artful, there's an existing test failure (20-live-
basic_die%%002-00001):
$ sudo apt build-dep -y libseccomp
$ sudo apt install -y cython
$ apt source libseccomp
$ cd libseccomp-*
$ autoreconf -ivf && ./configure --enable-python && make check-build
$ (cd tests && ./regression -T live)
All tests should pass on zesty (12 tests) and xenial (10 tests). On artful,
you'll see one pre-existing failure:
...
Test 20-live-basic_die%%002-00001 result: FAILURE 20-live-basic_die TRAP
rc=159
...
Regression Test Summary
tests run: 12
tests skipped: 0
tests passed: 11
tests failed: 1
tests errored: 0
============================================================
----------------------------
Running the seccomp kernel selftests is also a great to exercise
seccomp and the kernel patch set proposed for the SRU includes
additional seccomp selftests. To build, enter into the root of the
kernel source tree and build the seccomp test binary:
$ make -C tools/testing/selftests TARGETS=seccomp
Now you can execute tools/testing/selftests/seccomp/seccomp_bpf or
even copy it to a test machine and run it there. On Xenial, 54/54
tests should pass and 58/58 should pass on Zesty.
----------------------------
Now we can run a single test to verify that SECCOMP_RET_ERRNO is
logged when the application opts into it. First, verify that "errno"
is listed in the actions_logged sysctl:
$ cat /proc/sys/kernel/seccomp/actions_logged
kill trap errno trace log
Now, build and run the test program:
$ gcc -o lp1721676-kernel-test lp1721676-kernel-test.c
$ ./lp1721676-kernel-test
SUCCESS: getpid() failed as expected: Operation not permitted
It should have generated a message like this in /var/log/syslog:
kernel: [79338.804966] audit: type=1326 audit(1507259221.875:27):
auid=1000 uid=1000 gid=1000 ses=5 pid=3091 comm="lp1721676-kerne"
exe="/home/tyhicks/lp1721676-kernel-test" sig=0 arch=c000003e
syscall=39 compat=0 ip=0x7fb91829c499 code=0x50000
Disable errno logging in the sysctl:
$ echo kill trap trace log | sudo tee /proc/sys/kernel/seccomp/actions_logged
kill trap trace log
Rerun the test program and ensure that nothing was logged this time.
[Regression Potential]
The kernel patches received a lot of review between Kees and some
others interested in improved seccomp logging. I authored the patches
and feel comfortable/confident with my backported versions. They do
not change the behavior of seccomp logging by default but offer ways
applications to opt into more logging and, on the flipside, ways for
the administrator to quite any additional logging.
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