nptl: Move setxid broadcast implementation into libc

The signal handler is exported as __nptl_setxid_sighandler, so
that the libpthread initialization code can install it.  This
is sufficient for now because it is guarantueed to happen before
the first pthread_create call.

1 files changed, 279 insertions, 0 deletions

diff --git a/nptl/nptl_setxid.c b/nptl/nptl_setxid.c
new file mode 100644
index 0000000..2f35772
--- /dev/null
+++ b/nptl/nptl_setxid.c
@@ -0,0 +1,279 @@
+/* Copyright (C) 2002-2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
+
+   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
+   <https://www.gnu.org/licenses/>.  */
+
+#include <futex-internal.h>
+#include <ldsodefs.h>
+#include <list.h>
+#include <lowlevellock.h>
+#include <pthreadP.h>
+#include <unistd.h>
+
+/* Check for consistency across set*id system call results.  The abort
+   should not happen as long as all privileges changes happen through
+   the glibc wrappers.  ERROR must be 0 (no error) or an errno
+   code.  */
+static void
+setxid_error (struct xid_command *cmdp, int error)
+{
+  do
+    {
+      int olderror = cmdp->error;
+      if (olderror == error)
+        break;
+      if (olderror != -1)
+        {
+          /* Mismatch between current and previous results.  Save the
+             error value to memory so that is not clobbered by the
+             abort function and preserved in coredumps.  */
+          volatile int xid_err __attribute__ ((unused)) = error;
+          abort ();
+        }
+    }
+  while (atomic_compare_and_exchange_bool_acq (&cmdp->error, error, -1));
+}
+
+/* Set by __nptl_setxid and used by __nptl_setxid_sighandler.  */
+static struct xid_command *xidcmd;
+
+/* We use the SIGSETXID signal in the setuid, setgid, etc. implementations to
+   tell each thread to call the respective setxid syscall on itself.  This is
+   the handler.  */
+void
+__nptl_setxid_sighandler (int sig, siginfo_t *si, void *ctx)
+{
+  int result;
+
+  /* Safety check.  It would be possible to call this function for
+     other signals and send a signal from another process.  This is not
+     correct and might even be a security problem.  Try to catch as
+     many incorrect invocations as possible.  */
+  if (sig != SIGSETXID
+      || si->si_pid != __getpid ()
+      || si->si_code != SI_TKILL)
+    return;
+
+  result = INTERNAL_SYSCALL_NCS (xidcmd->syscall_no, 3, xidcmd->id[0],
+				 xidcmd->id[1], xidcmd->id[2]);
+  int error = 0;
+  if (__glibc_unlikely (INTERNAL_SYSCALL_ERROR_P (result)))
+    error = INTERNAL_SYSCALL_ERRNO (result);
+  setxid_error (xidcmd, error);
+
+  /* Reset the SETXID flag.  */
+  struct pthread *self = THREAD_SELF;
+  int flags, newval;
+  do
+    {
+      flags = THREAD_GETMEM (self, cancelhandling);
+      newval = THREAD_ATOMIC_CMPXCHG_VAL (self, cancelhandling,
+					  flags & ~SETXID_BITMASK, flags);
+    }
+  while (flags != newval);
+
+  /* And release the futex.  */
+  self->setxid_futex = 1;
+  futex_wake (&self->setxid_futex, 1, FUTEX_PRIVATE);
+
+  if (atomic_decrement_val (&xidcmd->cntr) == 0)
+    futex_wake ((unsigned int *) &xidcmd->cntr, 1, FUTEX_PRIVATE);
+}
+libc_hidden_def (__nptl_setxid_sighandler)
+
+static void
+setxid_mark_thread (struct xid_command *cmdp, struct pthread *t)
+{
+  int ch;
+
+  /* Wait until this thread is cloned.  */
+  if (t->setxid_futex == -1
+      && ! atomic_compare_and_exchange_bool_acq (&t->setxid_futex, -2, -1))
+    do
+      futex_wait_simple (&t->setxid_futex, -2, FUTEX_PRIVATE);
+    while (t->setxid_futex == -2);
+
+  /* Don't let the thread exit before the setxid handler runs.  */
+  t->setxid_futex = 0;
+
+  do
+    {
+      ch = t->cancelhandling;
+
+      /* If the thread is exiting right now, ignore it.  */
+      if ((ch & EXITING_BITMASK) != 0)
+        {
+          /* Release the futex if there is no other setxid in
+             progress.  */
+          if ((ch & SETXID_BITMASK) == 0)
+            {
+              t->setxid_futex = 1;
+              futex_wake (&t->setxid_futex, 1, FUTEX_PRIVATE);
+            }
+          return;
+        }
+    }
+  while (atomic_compare_and_exchange_bool_acq (&t->cancelhandling,
+                                               ch | SETXID_BITMASK, ch));
+}
+
+
+static void
+setxid_unmark_thread (struct xid_command *cmdp, struct pthread *t)
+{
+  int ch;
+
+  do
+    {
+      ch = t->cancelhandling;
+      if ((ch & SETXID_BITMASK) == 0)
+        return;
+    }
+  while (atomic_compare_and_exchange_bool_acq (&t->cancelhandling,
+                                               ch & ~SETXID_BITMASK, ch));
+
+  /* Release the futex just in case.  */
+  t->setxid_futex = 1;
+  futex_wake (&t->setxid_futex, 1, FUTEX_PRIVATE);
+}
+
+
+static int
+setxid_signal_thread (struct xid_command *cmdp, struct pthread *t)
+{
+  if ((t->cancelhandling & SETXID_BITMASK) == 0)
+    return 0;
+
+  int val;
+  pid_t pid = __getpid ();
+  val = INTERNAL_SYSCALL_CALL (tgkill, pid, t->tid, SIGSETXID);
+
+  /* If this failed, it must have had not started yet or else exited.  */
+  if (!INTERNAL_SYSCALL_ERROR_P (val))
+    {
+      atomic_increment (&cmdp->cntr);
+      return 1;
+    }
+  else
+    return 0;
+}
+
+int
+attribute_hidden
+__nptl_setxid (struct xid_command *cmdp)
+{
+  int signalled;
+  int result;
+  lll_lock (GL (dl_stack_cache_lock), LLL_PRIVATE);
+
+  xidcmd = cmdp;
+  cmdp->cntr = 0;
+  cmdp->error = -1;
+
+  struct pthread *self = THREAD_SELF;
+
+  /* Iterate over the list with system-allocated threads first.  */
+  list_t *runp;
+  list_for_each (runp, &GL (dl_stack_used))
+    {
+      struct pthread *t = list_entry (runp, struct pthread, list);
+      if (t == self)
+        continue;
+
+      setxid_mark_thread (cmdp, t);
+    }
+
+  /* Now the list with threads using user-allocated stacks.  */
+  list_for_each (runp, &GL (dl_stack_user))
+    {
+      struct pthread *t = list_entry (runp, struct pthread, list);
+      if (t == self)
+        continue;
+
+      setxid_mark_thread (cmdp, t);
+    }
+
+  /* Iterate until we don't succeed in signalling anyone.  That means
+     we have gotten all running threads, and their children will be
+     automatically correct once started.  */
+  do
+    {
+      signalled = 0;
+
+      list_for_each (runp, &GL (dl_stack_used))
+        {
+          struct pthread *t = list_entry (runp, struct pthread, list);
+          if (t == self)
+            continue;
+
+          signalled += setxid_signal_thread (cmdp, t);
+        }
+
+      list_for_each (runp, &GL (dl_stack_user))
+        {
+          struct pthread *t = list_entry (runp, struct pthread, list);
+          if (t == self)
+            continue;
+
+          signalled += setxid_signal_thread (cmdp, t);
+        }
+
+      int cur = cmdp->cntr;
+      while (cur != 0)
+        {
+          futex_wait_simple ((unsigned int *) &cmdp->cntr, cur,
+                             FUTEX_PRIVATE);
+          cur = cmdp->cntr;
+        }
+    }
+  while (signalled != 0);
+
+  /* Clean up flags, so that no thread blocks during exit waiting
+     for a signal which will never come.  */
+  list_for_each (runp, &GL (dl_stack_used))
+    {
+      struct pthread *t = list_entry (runp, struct pthread, list);
+      if (t == self)
+        continue;
+
+      setxid_unmark_thread (cmdp, t);
+    }
+
+  list_for_each (runp, &GL (dl_stack_user))
+    {
+      struct pthread *t = list_entry (runp, struct pthread, list);
+      if (t == self)
+        continue;
+
+      setxid_unmark_thread (cmdp, t);
+    }
+
+  /* This must be last, otherwise the current thread might not have
+     permissions to send SIGSETXID syscall to the other threads.  */
+  result = INTERNAL_SYSCALL_NCS (cmdp->syscall_no, 3,
+                                 cmdp->id[0], cmdp->id[1], cmdp->id[2]);
+  int error = 0;
+  if (__glibc_unlikely (INTERNAL_SYSCALL_ERROR_P (result)))
+    {
+      error = INTERNAL_SYSCALL_ERRNO (result);
+      __set_errno (error);
+      result = -1;
+    }
+  setxid_error (cmdp, error);
+
+  lll_unlock (GL (dl_stack_cache_lock), LLL_PRIVATE);
+  return result;
+}