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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Tue, 21 Jan 2020 09:55:53 -0800
Message-ID: <CAADnVQK-JzK-GUk4KOozn4c1xr=7TiCpB9Fi0QDC9nE6iVn8iQ@mail.gmail.com>
Subject: Re: [PATCH v5 01/10] capabilities: introduce CAP_PERFMON to kernel
 and user space
To:     Alexey Budankov <alexey.budankov@linux.intel.com>
Cc:     Stephen Smalley <sds@tycho.nsa.gov>,
        Peter Zijlstra <peterz@infradead.org>,
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On Tue, Jan 21, 2020 at 9:31 AM Alexey Budankov
<alexey.budankov@linux.intel.com> 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@linux.intel.com>
> >> ---
> >>   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.
>
> Some of ideas from v4 review.

well, in the requested changes form v4 I wrote:
return capable(CAP_PERFMON);
instead of
return false;

That's what Andy suggested earlier for CAP_BPF.
I think that should resolve Stephen's concern.