nptl/nptl_setxid.c

   1 /* Copyright (C) 2002-2022 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <https://www.gnu.org/licenses/>.  */
  17
  18 #include <futex-internal.h>
  19 #include <ldsodefs.h>
  20 #include <list.h>
  21 #include <lowlevellock.h>
  22 #include <pthreadP.h>
  23 #include <unistd.h>
  24
  25 /* Check for consistency across set*id system call results.  The abort
  26    should not happen as long as all privileges changes happen through
  27    the glibc wrappers.  ERROR must be 0 (no error) or an errno
  28    code.  */
  29 static void
  30 setxid_error (struct xid_command *cmdp, int error)
  31 {
  32   do
  33     {
  34       int olderror = cmdp->error;
  35       if (olderror == error)
  36         break;
  37       if (olderror != -1)
  38         {
  39           /* Mismatch between current and previous results.  Save the
  40              error value to memory so that is not clobbered by the
  41              abort function and preserved in coredumps.  */
  42           volatile int xid_err __attribute__ ((unused)) = error;
  43           abort ();
  44         }
  45     }
  46   while (atomic_compare_and_exchange_bool_acq (&cmdp->error, error, -1));
  47 }
  48
  49 /* Set by __nptl_setxid and used by __nptl_setxid_sighandler.  */
  50 static struct xid_command *xidcmd;
  51
  52 /* We use the SIGSETXID signal in the setuid, setgid, etc. implementations to
  53    tell each thread to call the respective setxid syscall on itself.  This is
  54    the handler.  */
  55 void
  56 __nptl_setxid_sighandler (int sig, siginfo_t *si, void *ctx)
  57 {
  58   int result;
  59
  60   /* Safety check.  It would be possible to call this function for
  61      other signals and send a signal from another process.  This is not
  62      correct and might even be a security problem.  Try to catch as
  63      many incorrect invocations as possible.  */
  64   if (sig != SIGSETXID
  65       || si->si_pid != __getpid ()
  66       || si->si_code != SI_TKILL)
  67     return;
  68
  69   result = INTERNAL_SYSCALL_NCS (xidcmd->syscall_no, 3, xidcmd->id[0],
  70                                  xidcmd->id[1], xidcmd->id[2]);
  71   int error = 0;
  72   if (__glibc_unlikely (INTERNAL_SYSCALL_ERROR_P (result)))
  73     error = INTERNAL_SYSCALL_ERRNO (result);
  74   setxid_error (xidcmd, error);
  75
  76   /* Reset the SETXID flag.  */
  77   struct pthread *self = THREAD_SELF;
  78   int flags, newval;
  79   do
  80     {
  81       flags = THREAD_GETMEM (self, cancelhandling);
  82       newval = THREAD_ATOMIC_CMPXCHG_VAL (self, cancelhandling,
  83                                           flags & ~SETXID_BITMASK, flags);
  84     }
  85   while (flags != newval);
  86
  87   /* And release the futex.  */
  88   self->setxid_futex = 1;
  89   futex_wake (&self->setxid_futex, 1, FUTEX_PRIVATE);
  90
  91   if (atomic_fetch_add_relaxed (&xidcmd->cntr, -1) == 1)
  92     futex_wake ((unsigned int *) &xidcmd->cntr, 1, FUTEX_PRIVATE);
  93 }
  94 libc_hidden_def (__nptl_setxid_sighandler)
  95
  96 static void
  97 setxid_mark_thread (struct xid_command *cmdp, struct pthread *t)
  98 {
  99   int ch;
 100
 101   /* Wait until this thread is cloned.  */
 102   if (t->setxid_futex == -1
 103       && ! atomic_compare_and_exchange_bool_acq (&t->setxid_futex, -2, -1))
 104     do
 105       futex_wait_simple (&t->setxid_futex, -2, FUTEX_PRIVATE);
 106     while (t->setxid_futex == -2);
 107
 108   /* Don't let the thread exit before the setxid handler runs.  */
 109   t->setxid_futex = 0;
 110
 111   do
 112     {
 113       ch = t->cancelhandling;
 114
 115       /* If the thread is exiting right now, ignore it.  */
 116       if ((ch & EXITING_BITMASK) != 0)
 117         {
 118           /* Release the futex if there is no other setxid in
 119              progress.  */
 120           if ((ch & SETXID_BITMASK) == 0)
 121             {
 122               t->setxid_futex = 1;
 123               futex_wake (&t->setxid_futex, 1, FUTEX_PRIVATE);
 124             }
 125           return;
 126         }
 127     }
 128   while (atomic_compare_and_exchange_bool_acq (&t->cancelhandling,
 129                                                ch | SETXID_BITMASK, ch));
 130 }
 131
 132
 133 static void
 134 setxid_unmark_thread (struct xid_command *cmdp, struct pthread *t)
 135 {
 136   int ch;
 137
 138   do
 139     {
 140       ch = t->cancelhandling;
 141       if ((ch & SETXID_BITMASK) == 0)
 142         return;
 143     }
 144   while (atomic_compare_and_exchange_bool_acq (&t->cancelhandling,
 145                                                ch & ~SETXID_BITMASK, ch));
 146
 147   /* Release the futex just in case.  */
 148   t->setxid_futex = 1;
 149   futex_wake (&t->setxid_futex, 1, FUTEX_PRIVATE);
 150 }
 151
 152
 153 static int
 154 setxid_signal_thread (struct xid_command *cmdp, struct pthread *t)
 155 {
 156   if ((t->cancelhandling & SETXID_BITMASK) == 0)
 157     return 0;
 158
 159   int val;
 160   pid_t pid = __getpid ();
 161   val = INTERNAL_SYSCALL_CALL (tgkill, pid, t->tid, SIGSETXID);
 162
 163   /* If this failed, it must have had not started yet or else exited.  */
 164   if (!INTERNAL_SYSCALL_ERROR_P (val))
 165     {
 166       atomic_fetch_add_relaxed (&cmdp->cntr, 1);
 167       return 1;
 168     }
 169   else
 170     return 0;
 171 }
 172
 173 int
 174 attribute_hidden
 175 __nptl_setxid (struct xid_command *cmdp)
 176 {
 177   int signalled;
 178   int result;
 179   lll_lock (GL (dl_stack_cache_lock), LLL_PRIVATE);
 180
 181   xidcmd = cmdp;
 182   cmdp->cntr = 0;
 183   cmdp->error = -1;
 184
 185   struct pthread *self = THREAD_SELF;
 186
 187   /* Iterate over the list with system-allocated threads first.  */
 188   list_t *runp;
 189   list_for_each (runp, &GL (dl_stack_used))
 190     {
 191       struct pthread *t = list_entry (runp, struct pthread, list);
 192       if (t == self)
 193         continue;
 194
 195       setxid_mark_thread (cmdp, t);
 196     }
 197
 198   /* Now the list with threads using user-allocated stacks.  */
 199   list_for_each (runp, &GL (dl_stack_user))
 200     {
 201       struct pthread *t = list_entry (runp, struct pthread, list);
 202       if (t == self)
 203         continue;
 204
 205       setxid_mark_thread (cmdp, t);
 206     }
 207
 208   /* Iterate until we don't succeed in signalling anyone.  That means
 209      we have gotten all running threads, and their children will be
 210      automatically correct once started.  */
 211   do
 212     {
 213       signalled = 0;
 214
 215       list_for_each (runp, &GL (dl_stack_used))
 216         {
 217           struct pthread *t = list_entry (runp, struct pthread, list);
 218           if (t == self)
 219             continue;
 220
 221           signalled += setxid_signal_thread (cmdp, t);
 222         }
 223
 224       list_for_each (runp, &GL (dl_stack_user))
 225         {
 226           struct pthread *t = list_entry (runp, struct pthread, list);
 227           if (t == self)
 228             continue;
 229
 230           signalled += setxid_signal_thread (cmdp, t);
 231         }
 232
 233       int cur = cmdp->cntr;
 234       while (cur != 0)
 235         {
 236           futex_wait_simple ((unsigned int *) &cmdp->cntr, cur,
 237                              FUTEX_PRIVATE);
 238           cur = cmdp->cntr;
 239         }
 240     }
 241   while (signalled != 0);
 242
 243   /* Clean up flags, so that no thread blocks during exit waiting
 244      for a signal which will never come.  */
 245   list_for_each (runp, &GL (dl_stack_used))
 246     {
 247       struct pthread *t = list_entry (runp, struct pthread, list);
 248       if (t == self)
 249         continue;
 250
 251       setxid_unmark_thread (cmdp, t);
 252     }
 253
 254   list_for_each (runp, &GL (dl_stack_user))
 255     {
 256       struct pthread *t = list_entry (runp, struct pthread, list);
 257       if (t == self)
 258         continue;
 259
 260       setxid_unmark_thread (cmdp, t);
 261     }
 262
 263   /* This must be last, otherwise the current thread might not have
 264      permissions to send SIGSETXID syscall to the other threads.  */
 265   result = INTERNAL_SYSCALL_NCS (cmdp->syscall_no, 3,
 266                                  cmdp->id[0], cmdp->id[1], cmdp->id[2]);
 267   int error = 0;
 268   if (__glibc_unlikely (INTERNAL_SYSCALL_ERROR_P (result)))
 269     {
 270       error = INTERNAL_SYSCALL_ERRNO (result);
 271       __set_errno (error);
 272       result = -1;
 273     }
 274   setxid_error (cmdp, error);
 275
 276   lll_unlock (GL (dl_stack_cache_lock), LLL_PRIVATE);
 277   return result;
 278 }