Linux: Implement interfaces for memory protection keys

This adds system call wrappers for pkey_alloc, pkey_free, pkey_mprotect,
and x86-64 implementations of pkey_get and pkey_set, which abstract over
the PKRU CPU register and hide the actual number of memory protection
keys supported by the CPU.  pkey_mprotect with a -1 key is implemented
using mprotect, so it will work even if the kernel does not support the
pkey_mprotect system call.

The system call wrapers use unsigned int instead of unsigned long for
parameters, so that no special treatment for x32 is needed.  The flags
argument is currently unused, and the access rights bit mask is limited
to two bits by the current PKRU register layout anyway.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

1 files changed, 399 insertions, 0 deletions

diff --git a/sysdeps/unix/sysv/linux/tst-pkey.c b/sysdeps/unix/sysv/linux/tst-pkey.c
new file mode 100644
index 0000000000..e7205dba1f
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/tst-pkey.c
@@ -0,0 +1,399 @@
+/* Tests for memory protection keys.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <inttypes.h>
+#include <setjmp.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <support/check.h>
+#include <support/support.h>
+#include <support/test-driver.h>
+#include <support/xsignal.h>
+#include <support/xthread.h>
+#include <support/xunistd.h>
+#include <sys/mman.h>
+
+/* Used to force threads to wait until the main thread has set up the
+   keys as intended.  */
+static pthread_barrier_t barrier;
+
+/* The keys used for testing.  These have been allocated with access
+   rights set based on their array index.  */
+enum { key_count = 4 };
+static int keys[key_count];
+static volatile int *pages[key_count];
+
+/* Used to report results from the signal handler.  */
+static volatile void *sigsegv_addr;
+static volatile int sigsegv_code;
+static volatile int sigsegv_pkey;
+static sigjmp_buf sigsegv_jmp;
+
+/* Used to handle expected read or write faults.  */
+static void
+sigsegv_handler (int signum, siginfo_t *info, void *context)
+{
+  sigsegv_addr = info->si_addr;
+  sigsegv_code = info->si_code;
+  sigsegv_pkey = info->si_pkey;
+  siglongjmp (sigsegv_jmp, 2);
+}
+
+static const struct sigaction sigsegv_sigaction =
+  {
+    .sa_flags = SA_RESETHAND | SA_SIGINFO,
+    .sa_sigaction = &sigsegv_handler,
+  };
+
+/* Check if PAGE is readable (if !WRITE) or writable (if WRITE).  */
+static bool
+check_page_access (int page, bool write)
+{
+  /* This is needed to work around bug 22396: On x86-64, siglongjmp
+     does not restore the protection key access rights for the current
+     thread.  We restore only the access rights for the keys under
+     test.  (This is not a general solution to this problem, but it
+     allows testing to proceed after a fault.)  */
+  unsigned saved_rights[key_count];
+  for (int i = 0; i < key_count; ++i)
+    saved_rights[i] = pkey_get (keys[i]);
+
+  volatile int *addr = pages[page];
+  if (test_verbose > 0)
+    {
+      printf ("info: checking access at %p (page %d) for %s\n",
+              addr, page, write ? "writing" : "reading");
+    }
+  int result = sigsetjmp (sigsegv_jmp, 1);
+  if (result == 0)
+    {
+      xsigaction (SIGSEGV, &sigsegv_sigaction, NULL);
+      if (write)
+        *addr = 3;
+      else
+        (void) *addr;
+      xsignal (SIGSEGV, SIG_DFL);
+      if (test_verbose > 0)
+        puts ("  --> access allowed");
+      return true;
+    }
+  else
+    {
+      xsignal (SIGSEGV, SIG_DFL);
+      if (test_verbose > 0)
+        puts ("  --> access denied");
+      TEST_COMPARE (result, 2);
+      TEST_COMPARE ((uintptr_t) sigsegv_addr, (uintptr_t) addr);
+      TEST_COMPARE (sigsegv_code, SEGV_PKUERR);
+      TEST_COMPARE (sigsegv_pkey, keys[page]);
+      for (int i = 0; i < key_count; ++i)
+        TEST_COMPARE (pkey_set (keys[i], saved_rights[i]), 0);
+      return false;
+    }
+}
+
+static volatile sig_atomic_t sigusr1_handler_ran;
+
+/* Used to check that access is revoked in signal handlers.  */
+static void
+sigusr1_handler (int signum)
+{
+  TEST_COMPARE (signum, SIGUSR1);
+  for (int i = 0; i < key_count; ++i)
+    TEST_COMPARE (pkey_get (keys[i]), PKEY_DISABLE_ACCESS);
+  sigusr1_handler_ran = 1;
+}
+
+/* Used to report results from other threads.  */
+struct thread_result
+{
+  int access_rights[key_count];
+  pthread_t next_thread;
+};
+
+/* Return the thread's access rights for the keys under test.  */
+static void *
+get_thread_func (void *closure)
+{
+  struct thread_result *result = xmalloc (sizeof (*result));
+  for (int i = 0; i < key_count; ++i)
+    result->access_rights[i] = pkey_get (keys[i]);
+  memset (&result->next_thread, 0, sizeof (result->next_thread));
+  return result;
+}
+
+/* Wait for initialization and then check that the current thread does
+   not have access through the keys under test.  */
+static void *
+delayed_thread_func (void *closure)
+{
+  bool check_access = *(bool *) closure;
+  pthread_barrier_wait (&barrier);
+  struct thread_result *result = get_thread_func (NULL);
+
+  if (check_access)
+    {
+      /* Also check directly.  This code should not run with other
+         threads in parallel because of the SIGSEGV handler which is
+         installed by check_page_access.  */
+      for (int i = 0; i < key_count; ++i)
+        {
+          TEST_VERIFY (!check_page_access (i, false));
+          TEST_VERIFY (!check_page_access (i, true));
+        }
+    }
+
+  result->next_thread = xpthread_create (NULL, get_thread_func, NULL);
+  return result;
+}
+
+static int
+do_test (void)
+{
+  long pagesize = xsysconf (_SC_PAGESIZE);
+
+  /* pkey_mprotect with key -1 should work even when there is no
+     protection key support.  */
+  {
+    int *page = xmmap (NULL, pagesize, PROT_NONE,
+                       MAP_ANONYMOUS | MAP_PRIVATE, -1);
+    TEST_COMPARE (pkey_mprotect (page, pagesize, PROT_READ | PROT_WRITE, -1),
+                  0);
+    volatile int *vpage = page;
+    *vpage = 5;
+    TEST_COMPARE (*vpage, 5);
+    xmunmap (page, pagesize);
+  }
+
+  xpthread_barrier_init (&barrier, NULL, 2);
+  bool delayed_thread_check_access = true;
+  pthread_t delayed_thread = xpthread_create
+    (NULL, &delayed_thread_func, &delayed_thread_check_access);
+
+  keys[0] = pkey_alloc (0, 0);
+  if (keys[0] < 0)
+    {
+      if (errno == ENOSYS)
+        FAIL_UNSUPPORTED
+          ("kernel does not support memory protection keys");
+      if (errno == EINVAL)
+        FAIL_UNSUPPORTED
+          ("CPU does not support memory protection keys: %m");
+      FAIL_EXIT1 ("pkey_alloc: %m");
+    }
+  TEST_COMPARE (pkey_get (keys[0]), 0);
+  for (int i = 1; i < key_count; ++i)
+    {
+      keys[i] = pkey_alloc (0, i);
+      if (keys[i] < 0)
+        FAIL_EXIT1 ("pkey_alloc (0, %d): %m", i);
+      /* pkey_alloc is supposed to change the current thread's access
+         rights for the new key.  */
+      TEST_COMPARE (pkey_get (keys[i]), i);
+    }
+  /* Check that all the keys have the expected access rights for the
+     current thread.  */
+  for (int i = 0; i < key_count; ++i)
+    TEST_COMPARE (pkey_get (keys[i]), i);
+
+  /* Allocate a test page for each key.  */
+  for (int i = 0; i < key_count; ++i)
+    {
+      pages[i] = xmmap (NULL, pagesize, PROT_READ | PROT_WRITE,
+                        MAP_ANONYMOUS | MAP_PRIVATE, -1);
+      TEST_COMPARE (pkey_mprotect ((void *) pages[i], pagesize,
+                                   PROT_READ | PROT_WRITE, keys[i]), 0);
+    }
+
+  /* Check that the initial thread does not have access to the new
+     keys.  */
+  {
+    pthread_barrier_wait (&barrier);
+    struct thread_result *result = xpthread_join (delayed_thread);
+    for (int i = 0; i < key_count; ++i)
+      TEST_COMPARE (result->access_rights[i],
+                    PKEY_DISABLE_ACCESS);
+    struct thread_result *result2 = xpthread_join (result->next_thread);
+    for (int i = 0; i < key_count; ++i)
+      TEST_COMPARE (result->access_rights[i],
+                    PKEY_DISABLE_ACCESS);
+    free (result);
+    free (result2);
+  }
+
+  /* Check that the current thread access rights are inherited by new
+     threads.  */
+  {
+    pthread_t get_thread = xpthread_create (NULL, get_thread_func, NULL);
+    struct thread_result *result = xpthread_join (get_thread);
+    for (int i = 0; i < key_count; ++i)
+      TEST_COMPARE (result->access_rights[i], i);
+    free (result);
+  }
+
+  for (int i = 0; i < key_count; ++i)
+    TEST_COMPARE (pkey_get (keys[i]), i);
+
+  /* Check that in a signal handler, there is no access.  */
+  xsignal (SIGUSR1, &sigusr1_handler);
+  xraise (SIGUSR1);
+  xsignal (SIGUSR1, SIG_DFL);
+  TEST_COMPARE (sigusr1_handler_ran, 1);
+
+  /* The first key results in a writable page.  */
+  TEST_VERIFY (check_page_access (0, false));
+  TEST_VERIFY (check_page_access (0, true));
+
+  /* The other keys do not.   */
+  for (int i = 1; i < key_count; ++i)
+    {
+      if (test_verbose)
+        printf ("info: checking access for key %d, bits 0x%x\n",
+                i, pkey_get (keys[i]));
+      for (int j = 0; j < key_count; ++j)
+        TEST_COMPARE (pkey_get (keys[j]), j);
+      if (i & PKEY_DISABLE_ACCESS)
+        {
+          TEST_VERIFY (!check_page_access (i, false));
+          TEST_VERIFY (!check_page_access (i, true));
+        }
+      else
+        {
+          TEST_VERIFY (i & PKEY_DISABLE_WRITE);
+          TEST_VERIFY (check_page_access (i, false));
+          TEST_VERIFY (!check_page_access (i, true));
+        }
+    }
+
+  /* But if we set the current thread's access rights, we gain
+     access.  */
+  for (int do_write = 0; do_write < 2; ++do_write)
+    for (int allowed_key = 0; allowed_key < key_count; ++allowed_key)
+      {
+        for (int i = 0; i < key_count; ++i)
+          if (i == allowed_key)
+            {
+              if (do_write)
+                TEST_COMPARE (pkey_set (keys[i], 0), 0);
+              else
+                TEST_COMPARE (pkey_set (keys[i], PKEY_DISABLE_WRITE), 0);
+            }
+          else
+            TEST_COMPARE (pkey_set (keys[i], PKEY_DISABLE_ACCESS), 0);
+
+        if (test_verbose)
+          printf ("info: key %d is allowed access for %s\n",
+                  allowed_key, do_write ? "writing" : "reading");
+        for (int i = 0; i < key_count; ++i)
+          if (i == allowed_key)
+            {
+              TEST_VERIFY (check_page_access (i, false));
+              TEST_VERIFY (check_page_access (i, true) == do_write);
+            }
+          else
+            {
+              TEST_VERIFY (!check_page_access (i, false));
+              TEST_VERIFY (!check_page_access (i, true));
+            }
+      }
+
+  /* Restore access to all keys, and launch a thread which should
+     inherit that access.  */
+  for (int i = 0; i < key_count; ++i)
+    {
+      TEST_COMPARE (pkey_set (keys[i], 0), 0);
+      TEST_VERIFY (check_page_access (i, false));
+      TEST_VERIFY (check_page_access (i, true));
+    }
+  delayed_thread_check_access = false;
+  delayed_thread = xpthread_create
+    (NULL, delayed_thread_func, &delayed_thread_check_access);
+
+  TEST_COMPARE (pkey_free (keys[0]), 0);
+  /* Second pkey_free will fail because the key has already been
+     freed.  */
+  TEST_COMPARE (pkey_free (keys[0]),-1);
+  TEST_COMPARE (errno, EINVAL);
+  for (int i = 1; i < key_count; ++i)
+    TEST_COMPARE (pkey_free (keys[i]), 0);
+
+  /* Check what happens to running threads which have access to
+     previously allocated protection keys.  The implemented behavior
+     is somewhat dubious: Ideally, pkey_free should revoke access to
+     that key and pkey_alloc of the same (numeric) key should not
+     implicitly confer access to already-running threads, but this is
+     not what happens in practice.  */
+  {
+    /* The limit is in place to avoid running indefinitely in case
+       there many keys available.  */
+    int *keys_array = xcalloc (100000, sizeof (*keys_array));
+    int keys_allocated = 0;
+    while (keys_allocated < 100000)
+      {
+        int new_key = pkey_alloc (0, PKEY_DISABLE_WRITE);
+        if (new_key < 0)
+          {
+            /* No key reuse observed before running out of keys.  */
+            TEST_COMPARE (errno, ENOSPC);
+            break;
+          }
+        for (int i = 0; i < key_count; ++i)
+          if (new_key == keys[i])
+            {
+              /* We allocated the key with disabled write access.
+                 This should affect the protection state of the
+                 existing page.  */
+              TEST_VERIFY (check_page_access (i, false));
+              TEST_VERIFY (!check_page_access (i, true));
+
+              xpthread_barrier_wait (&barrier);
+              struct thread_result *result = xpthread_join (delayed_thread);
+              /* The thread which was launched before should still have
+                 access to the key.  */
+              TEST_COMPARE (result->access_rights[i], 0);
+              struct thread_result *result2
+                = xpthread_join (result->next_thread);
+              /* Same for a thread which is launched afterwards from
+                 the old thread.  */
+              TEST_COMPARE (result2->access_rights[i], 0);
+              free (result);
+              free (result2);
+              keys_array[keys_allocated++] = new_key;
+              goto after_key_search;
+            }
+        /* Save key for later deallocation.  */
+        keys_array[keys_allocated++] = new_key;
+      }
+  after_key_search:
+    /* Deallocate the keys allocated for testing purposes.  */
+    for (int j = 0; j < keys_allocated; ++j)
+      TEST_COMPARE (pkey_free (keys_array[j]), 0);
+    free (keys_array);
+  }
+
+  for (int i = 0; i < key_count; ++i)
+    xmunmap ((void *) pages[i], pagesize);
+
+  xpthread_barrier_destroy (&barrier);
+  return 0;
+}
+
+#include <support/test-driver.c>