Re: [RFC 0/5] perf: Per PMU access controls (paranoid setting)

[Alexey Budankov]

Hello,

On 01.10.2018 19:11, Thomas Gleixner wrote:

<SNIP>

> Peter and I discussed that and we came up with the idea that the file
> descriptor is not even required, i.e. you could make it backward
> compatible.
> 
> perf_event_open() knows which PMU is associated with the event the caller
> tries to open. So perf_event_open() can try to access/open the special per
> PMU file on behalf of the caller. That should get the same security
> treatment like a regular open() from user space. If that succeeds, access
> is granted.
> 
> The magic file could still be writeable for root to give general
> restrictions aside of the file based ones similar to what you are
> proposing.

Let me wrap up all the requirements and ideas that have been captured so far.

1. A file [1] is added so that it can belong to a group of users allowed to use ${PMU}, 
   something like this:

ls -alh /sys/bus/event_source/devices/${PMU}/caps/
total 0
drwxr-xr-x 2 root root            0 Oct  1 20:36 .
drwxr-xr-x 6 root root            0 Oct  1 20:36 ..
-r--r--r-- 1 root root         4.0K Oct  1 20:36 branches
-r--r--r-- 1 root root         4.0K Oct  1 20:36 max_precise
-r--r--r-- 1 root root         4.0K Oct  1 20:36 pmu_name
-rw-r--r--   root ${PMU}_users                   paranoid        <===

   Modifications of file content are allowed to those who can 
   modify /proc/sys/kernel/perf_event_paranoid setting.

2. Semantics and content of the introduced paranoid file is 
   similar to /proc/sys/kernel/perf_even_paranoid [2]:

   The perf_event_paranoid file can be set to restrict access
   to the performance counters.

   2   allow only user-space measurements (default since Linux 4.6).
   1   allow both kernel and user measurements (default before Linux 4.6).
   0   allow access to CPU-specific data but not raw traceâpoint samples.
  -1  no restrictions.

   The existence of the perf_event_paranoid file is the official method 
   for determining if a kernel supports perf_event_open().

3. Every time an event for ${PMU} is created over perf_event_open():
   a) the calling thread's euid is checked to belong to ${PMU}_users group 
      and if it does then the event's fd is allocated;
   b) then traditional checks against perf_event_pranoid content are applied;
   c) if the file doesn't exist the access is governed by global setting 
      at /proc/sys/kernel/perf_even_paranoid;

4. Documentation/admin-guide/perf-security.rst file is introduced that:
   a) contains general explanation for fine grained access control;
   b) contains a section with guidance about scope and risk for each PMU
      which is enabled for fine grained access control;
   c) file is extended when more PMUs are enabled for fine grain control;

> 
> The analysis and documentation requirements still remain of course.

Security analysis for uncore IMC, QPI/UPI, PCIe PMUs is still required 
to be enabled for fine grain control.

Thanks,
Alexey

[1] https://patchwork.kernel.org/patch/9249919/#19714087
[2] http://man7.org/linux/man-pages/man2/perf_event_open.2.html