pkeys on POWER: Access rights not reset on execve

[Florian Weimer <fweimer@xxxxxxxxxx>]

This test program:

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/syscall.h>
#include <err.h>

/* Return the value of the AMR register.  */
static inline unsigned long int
pkey_read (void)
{
  unsigned long int result;
  __asm__ volatile ("mfspr %0, 13" : "=r" (result));
  return result;
}

/* Overwrite the AMR register with VALUE.  */
static inline void
pkey_write (unsigned long int value)
{
  __asm__ volatile ("mtspr 13, %0" : : "r" (value));
}

int
main (int argc, char **argv)
{
  printf ("AMR (PID %d): 0x%016lx\n", (int) getpid (), pkey_read());
  if (argc > 1)
    {
      int key = syscall (__NR_pkey_alloc, 0, 0);
      if (key < 0)
        err (1, "pkey_alloc");
      printf ("Allocated key (PID %d): %d\n", (int) getpid (), key);
      return 0;
    }

  pid_t pid = fork ();
  if (pid == 0)
    {
      execl ("/proc/self/exe", argv[0], "subprocess", NULL);
      _exit (1);
    }
  if (pid < 0)
    err (1, "fork");
  int status;
  if (waitpid (pid, &status, 0) < 0)
    err (1, "waitpid");

  int key = syscall (__NR_pkey_alloc, 0, 0);
  if (key < 0)
    err (1, "pkey_alloc");
  printf ("Allocated key (PID %d): %d\n", (int) getpid (), key);

  unsigned long int amr = -1;
  printf ("Setting AMR: 0x%016lx\n", amr);
  pkey_write (amr);
  printf ("New AMR value (PID %d, before execl): 0x%016lx\n",
          (int) getpid (), pkey_read());
  execl ("/proc/self/exe", argv[0], "subprocess", NULL);
  err (1, "exec");
  return 1;
}

shows that the AMR register value is not reset on execve:

AMR (PID 112291): 0x0000000000000000
AMR (PID 112292): 0x0000000000000000
Allocated key (PID 112292): 2
Allocated key (PID 112291): 2
Setting AMR: 0xffffffffffffffff
New AMR value (PID 112291, before execl): 0x0c00000000000000
AMR (PID 112291): 0x0c00000000000000
Allocated key (PID 112291): 2

I think this is a real bug and needs to be fixed even if the defaults 
are kept as-is (see the other thread).

(Seen on 4.17.0-rc5.)

Thanks,
Florian