Re: [PATCH v5 01/10] capabilities: introduce CAP_PERFMON to kernel and user space

[Stephen Smalley]

On 1/22/20 5:45 AM, Alexey Budankov wrote:
> On 21.01.2020 21:27, Alexey Budankov wrote:
> > On 21.01.2020 20:55, Alexei Starovoitov wrote:
> > > On Tue, Jan 21, 2020 at 9:31 AM Alexey Budankov
> > > <alexey.budankov@xxxxxxxxxxxxxxx> wrote:
> > > > On 21.01.2020 17:43, Stephen Smalley wrote:
> > > > > On 1/20/20 6:23 AM, Alexey Budankov wrote:
> > > > > > Introduce CAP_PERFMON capability designed to secure system performance
> > > > > > monitoring and observability operations so that CAP_PERFMON would assist
> > > > > > CAP_SYS_ADMIN capability in its governing role for perf_events, i915_perf
> > > > > > and other performance monitoring and observability subsystems.
> > > > > >
> > > > > > CAP_PERFMON intends to harden system security and integrity during system
> > > > > > performance monitoring and observability operations by decreasing attack
> > > > > > surface that is available to a CAP_SYS_ADMIN privileged process [1].
> > > > > > Providing access to system performance monitoring and observability
> > > > > > operations under CAP_PERFMON capability singly, without the rest of
> > > > > > CAP_SYS_ADMIN credentials, excludes chances to misuse the credentials and
> > > > > > makes operation more secure.
> > > > > >
> > > > > > CAP_PERFMON intends to take over CAP_SYS_ADMIN credentials related to
> > > > > > system performance monitoring and observability operations and balance
> > > > > > amount of CAP_SYS_ADMIN credentials following the recommendations in the
> > > > > > capabilities man page [1] for CAP_SYS_ADMIN: "Note: this capability is
> > > > > > overloaded; see Notes to kernel developers, below."
> > > > > >
> > > > > > Although the software running under CAP_PERFMON can not ensure avoidance
> > > > > > of related hardware issues, the software can still mitigate these issues
> > > > > > following the official embargoed hardware issues mitigation procedure [2].
> > > > > > The bugs in the software itself could be fixed following the standard
> > > > > > kernel development process [3] to maintain and harden security of system
> > > > > > performance monitoring and observability operations.
> > > > > >
> > > > > > [1] http://man7.org/linux/man-pages/man7/capabilities.7.html
> > > > > > [2] https://www.kernel.org/doc/html/latest/process/embargoed-hardware-issues.html
> > > > > > [3] https://www.kernel.org/doc/html/latest/admin-guide/security-bugs.html
> > > > > >
> > > > > > Signed-off-by: Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx>
> > > > > > ---
> > > > > >    include/linux/capability.h          | 12 ++++++++++++
> > > > > >    include/uapi/linux/capability.h     |  8 +++++++-
> > > > > >    security/selinux/include/classmap.h |  4 ++--
> > > > > >    3 files changed, 21 insertions(+), 3 deletions(-)
> > > > > >
> > > > > > diff --git a/include/linux/capability.h b/include/linux/capability.h
> > > > > > index ecce0f43c73a..8784969d91e1 100644
> > > > > > --- a/include/linux/capability.h
> > > > > > +++ b/include/linux/capability.h
> > > > > > @@ -251,6 +251,18 @@ extern bool privileged_wrt_inode_uidgid(struct user_namespace *ns, const struct
> > > > > >    extern bool capable_wrt_inode_uidgid(const struct inode *inode, int cap);
> > > > > >    extern bool file_ns_capable(const struct file *file, struct user_namespace *ns, int cap);
> > > > > >    extern bool ptracer_capable(struct task_struct *tsk, struct user_namespace *ns);
> > > > > > +static inline bool perfmon_capable(void)
> > > > > > +{
> > > > > > +    struct user_namespace *ns = &init_user_ns;
> > > > > > +
> > > > > > +    if (ns_capable_noaudit(ns, CAP_PERFMON))
> > > > > > +        return ns_capable(ns, CAP_PERFMON);
> > > > > > +
> > > > > > +    if (ns_capable_noaudit(ns, CAP_SYS_ADMIN))
> > > > > > +        return ns_capable(ns, CAP_SYS_ADMIN);
> > > > > > +
> > > > > > +    return false;
> > > > > > +}
> > > > >
> > > > > Why _noaudit()?  Normally only used when a permission failure is non-fatal to the operation.  Otherwise, we want the audit message.
>
> So far so good, I suggest using the simplest version for v6:
>
> static inline bool perfmon_capable(void)
> {
> 	return capable(CAP_PERFMON) || capable(CAP_SYS_ADMIN);
> }
>
> It keeps the implementation simple and readable. The implementation is more
> performant in the sense of calling the API - one capable() call for CAP_PERFMON
> privileged process.
>
> Yes, it bloats audit log for CAP_SYS_ADMIN privileged and unprivileged processes,
> but this bloating also advertises and leverages using more secure CAP_PERFMON
> based approach to use perf_event_open system call.

I can live with that.  We just need to document that when you see both a 
CAP_PERFMON and a CAP_SYS_ADMIN audit message for a process, try only 
allowing CAP_PERFMON first and see if that resolves the issue.  We have 
a similar issue with CAP_DAC_READ_SEARCH versus CAP_DAC_OVERRIDE.