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

On 22.01.2020 17:25, Alexey Budankov wrote:

On 22.01.2020 17:07, Stephen Smalley wrote:

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On 1/22/20 5:45 AM, Alexey Budankov wrote:

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On 21.01.2020 21:27, Alexey Budankov wrote:

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On 21.01.2020 20:55, Alexei Starovoitov wrote:

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On Tue, Jan 21, 2020 at 9:31 AM Alexey Budankov
[off-list ref] wrote:

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On 21.01.2020 17:43, Stephen Smalley wrote:

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On 1/20/20 6:23 AM, Alexey Budankov wrote:

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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

<SNIP>

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Signed-off-by: Alexey Budankov <redacted>

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.

perf security [1] document can be updated, at least, to align and document 
this audit logging specifics.

And I plan to update the document right after this patch set is accepted.
Feel free to let me know of the places in the kernel docs that also
require update w.r.t CAP_PERFMON extension.

~Alexey

~Alexey

[1] https://www.kernel.org/doc/html/latest/admin-guide/perf-security.html