Linux: Fixes for getrandom fork handling

Careful updates of grnd_alloc.len are required to ensure that
after fork, grnd_alloc.states does not contain entries that
are also encountered by __getrandom_reset_state in TCBs.
For the same reason, it is necessary to overwrite the TCB state
pointer with NULL before updating grnd_alloc.states in
__getrandom_vdso_release.

Before this change, different TCBs could share the same getrandom
state after multi-threaded fork.  This would be a critical security
bug (predictable randomness) if not caught during development.

The additional check in stdlib/tst-arc4random-thread makes it more
likely that the test fails due to the bugs mentioned above.

Both __getrandom_reset_state and __getrandom_vdso_release could
put reserved NULL pointers into the states array.  This is also
fixed with this commit.  After these changes, no null pointers were
observed in the states array during testing.

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

1 files changed, 23 insertions, 3 deletions

diff --git a/sysdeps/unix/sysv/linux/getrandom.c b/sysdeps/unix/sysv/linux/getrandom.c
index eab2b68..6b7be5e 100644
--- a/sysdeps/unix/sysv/linux/getrandom.c
+++ b/sysdeps/unix/sysv/linux/getrandom.c
@@ -168,6 +168,11 @@ vgetrandom_get_state (void)
   if (grnd_alloc.len > 0 || vgetrandom_get_state_alloc ())
     state = grnd_alloc.states[--grnd_alloc.len];
 
+  /* Barrier needed by fork: The state must be gone from the array
+     through len update before it becomes visible in the TCB.  (There
+     is also a release barrier implied by the unlock, but issue a
+     stronger barrier to help fork.)  */
+  atomic_thread_fence_seq_cst ();
   __libc_lock_unlock (grnd_alloc.lock);
   internal_signal_restore_set (&set);
 
@@ -278,7 +283,10 @@ void
 __getrandom_reset_state (struct pthread *curp)
 {
 #ifdef HAVE_GETRANDOM_VSYSCALL
-  if (grnd_alloc.states == NULL || curp->getrandom_buf == NULL)
+  /* The pointer can be reserved if the fork happened during a
+     getrandom call.  */
+  void *buf = release_ptr (curp->getrandom_buf);
+  if (grnd_alloc.states == NULL || buf == NULL)
     return;
   assert (grnd_alloc.len < grnd_alloc.cap);
   grnd_alloc.states[grnd_alloc.len++] = release_ptr (curp->getrandom_buf);
@@ -294,11 +302,23 @@ void
 __getrandom_vdso_release (struct pthread *curp)
 {
 #ifdef HAVE_GETRANDOM_VSYSCALL
-  if (curp->getrandom_buf == NULL)
+  /* The pointer can be reserved if the thread was canceled in a
+     signal handler.  */
+  void *buf = release_ptr (curp->getrandom_buf);
+  if (buf == NULL)
     return;
 
   __libc_lock_lock (grnd_alloc.lock);
-  grnd_alloc.states[grnd_alloc.len++] = curp->getrandom_buf;
+
+  size_t len = grnd_alloc.len;
+  grnd_alloc.states[len] = curp->getrandom_buf;
+  curp->getrandom_buf = NULL;
+  /* Barrier needed by fork: The state must vanish from the TCB before
+     it becomes visible in the states array.  Also avoid exposing the
+     previous entry value at the same index in the states array (which
+     may be in use by another thread).  */
+  atomic_thread_fence_seq_cst ();
+  grnd_alloc.len = len + 1;
   __libc_lock_unlock (grnd_alloc.lock);
 #endif
 }