Re: CVS commit: src/lib/librt

[Thomas Klausner <wiz%NetBSD.org@localhost>]

On Mon, Feb 24, 2025 at 12:28:22AM +0100, Joerg Sonnenberger wrote:
> 
> 
> On 2/23/25 8:34 PM, Thomas Klausner wrote:
> > On Mon, Feb 24, 2025 at 06:32:09AM +1100, Matthew Green wrote:
> > > > Log Message:
> > > > Add LIBRARY section and explicitly mention that these interfaces
> > > > require super-user privileges.
> > > 
> > > this can be relaxed with the
> > > security.models.extensions.user_set_cpu_affinity sysctl.
> > 
> > I found this and wondered what the attack model is that makes it default to off...
> 
> Any kind of cache side channel attacks are much easier if you can explicitly
> control where things are executed.

Thanks for the explanation.

I just checked the man page again and noticed that you had been
talking about pset while I had thought we talked about the cpuset_*
interfaces.

I think the sysctl only affects the cpuset_* and not pset. At least
the attached test program, basically copied from the man page, fails
for me:

# sysctl security.models.extensions.user_set_cpu_affinity
security.models.extensions.user_set_cpu_affinity = 1
# gcc -Wall -o psettest psettest.c -lrt
# ./psettest
psettest: pset_create: Operation not permitted

 Thomas

#include <stdlib.h>
#include <stddef.h>
#include <stdio.h>
#include <sys/pset.h>
#include <err.h>

int main() {
             psetid_t psid;
             cpuid_t ci = 0;

             if (pset_create(&psid) < 0)
                     err(EXIT_FAILURE, "pset_create");

             /* Assign CPU 0 to the processor-set */
             if (pset_assign(psid, ci, NULL) < 0)
                     err(EXIT_FAILURE, "pset_assign");

             /* Bind the current process to the processor-set */
             if (pset_bind(psid, P_PID, P_MYID, NULL) < 0)
                     err(EXIT_FAILURE, "pset_bind");

             /*
              * At this point, CPU 0 runs only the current process.
              */
             printf("success\n");

             if (pset_destroy(psid) < 0)
                     err(EXIT_FAILURE, "pset_destroy");
}