On Sat, Jan 18, 2020 at 02:17:01AM +0100, Christian Brauner wrote:
On Fri, Jan 17, 2020 at 05:08:14PM -0800, Andrei Vagin wrote:
On Wed, Jan 15, 2020 at 9:18 AM Christian Brauner christian.brauner@ubuntu.com wrote:
Commit 69f594a38967 ("ptrace: do not audit capability check when outputing /proc/pid/stat") introduced the ability to opt out of audit messages for accesses to various proc files since they are not violations of policy. While doing so it somehow switched the check from ns_capable() to has_ns_capability{_noaudit}(). That means it switched from checking the subjective credentials of the task to using the objective credentials. I couldn't find the original lkml thread and so I don't know why this switch was done. But it seems wrong since ptrace_has_cap() is currently only used in ptrace_may_access(). And it's used to check whether the calling task (subject) has the CAP_SYS_PTRACE capability in the provided user namespace to operate on the target task (object). According to the cred.h comments this would mean the subjective credentials of the calling task need to be used. This switches it to use security_capable() because we only call ptrace_has_cap() in ptrace_may_access() and in there we already have a stable reference to the calling tasks creds under cred_guard_mutex so there's no need to go through another series of dereferences and rcu locking done in ns_capable{_noaudit}().
The criu process is started with all capabilities in the root user namespace.
I don't have time to investigate this issue right now, will provide more details next Tuesday.
Yeah, we've detected the issue. security_capable() indicates success by returning 0 for whatever reason whereas has_ns_capability() returns 1. So the logic was inverted. This is fixed in the new version. Sorry for the noise!
So, I just finished compiling criu and running the test suite on the criu-dev branch. The test-suite passes fine after the security_capable() braino in my original patch was corrected to security_capable() == 0:
################## ALL TEST(S) PASSED (TOTAL 178/SKIPPED 16) ###################
Thanks! Christian