elf: Assign TLS modid later during dlopen [BZ #24930]

Commit a42faf59d6 ("Fix BZ #16634.")
attempted to fix a TLS modid consistency issue by adding additional
checks to the open_verify function.  However, this is fragile
because open_verify cannot reliably predict whether
_dl_map_object_from_fd will later fail in the more complex cases
(such as memory allocation failures).  Therefore, this commit
assigns the TLS modid as late as possible.  At that point, the link
map pointer will eventually be passed to _dl_close, which will undo
the TLS modid assignment.

Reviewed-by: Gabriel F. T. Gomes <gabrielftg@linux.ibm.com>

ChangeLog

@@ -1,3 +1,15 @@
+2019-10-04  Florian Weimer  <fweimer@redhat.com>
+
+	[BZ #24930]
+	* elf/dl-load.c (_dl_map_object_from_fd): Only assign TLS modid if
+	the link map will be returned to the caller.
+	* elf/Makefile [$(have-fpie) && $(build-shared)] (tests, tests-pie):
+	Add tst-dlopen-aout-pie.
+	(tst-tst-dlopen-aout-no-pie): Set.
+	(CFLAGS-tst-dlopen-aout-pie.c): Build with -fpie.
+	(tst-dlopen-aout-pie): Link with -ldl -lpthread.
+	* elf/tst-dlopen-aout-pie.c: New file.
+
 2019-10-04  Florian Weimer  <fweimer@redhat.com>
 
 	[BZ #24900]

elf/Makefile

@@ -308,8 +308,8 @@ test-xfail-tst-protected1b = yes
 endif
 ifeq (yesyes,$(have-fpie)$(build-shared))
 modules-names += tst-piemod1
-tests += tst-pie1 tst-pie2 tst-dlopen-pie
+tests += tst-pie1 tst-pie2 tst-dlopen-pie tst-dlopen-aout-pie
-tests-pie += tst-pie1 tst-pie2
+tests-pie += tst-pie1 tst-pie2 tst-dlopen-aout-pie
 ifeq (yes,$(have-protected-data))
 tests += vismain
 tests-pie += vismain
@@ -1267,7 +1267,11 @@ tst-leaks1-static-ENV = MALLOC_TRACE=$(objpfx)tst-leaks1-static.mtrace
 $(objpfx)tst-addr1: $(libdl)
 
 $(objpfx)tst-thrlock: $(libdl) $(shared-thread-library)
+
+tst-tst-dlopen-aout-no-pie = yes
 $(objpfx)tst-dlopen-aout: $(libdl) $(shared-thread-library)
+CFLAGS-tst-dlopen-aout-pie.c += $(pie-ccflag)
+$(objpfx)tst-dlopen-aout-pie: $(libdl) $(shared-thread-library)
 $(objpfx)tst-dlopen-aout-container: $(libdl) $(shared-thread-library)
 LDFLAGS-tst-dlopen-aout-container += -Wl,-rpath,\$$ORIGIN
 

elf/dl-load.c

@@ -1118,27 +1118,21 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
 	     offset.  We will adjust it later.  */
 	  l->l_tls_initimage = (void *) ph->p_vaddr;
 
-	  /* If not loading the initial set of shared libraries,
+	  /* l->l_tls_modid is assigned below, once there is no
-	     check whether we should permit loading a TLS segment.  */
+	     possibility for failure.  */
-	  if (__glibc_likely (l->l_type == lt_library)
-	      /* If GL(dl_tls_dtv_slotinfo_list) == NULL, then rtld.c did
-		 not set up TLS data structures, so don't use them now.  */
-	      || __glibc_likely (GL(dl_tls_dtv_slotinfo_list) != NULL))
-	    {
-	      /* Assign the next available module ID.  */
-	      l->l_tls_modid = _dl_next_tls_modid ();
-	      break;
-	    }
 
+	  if (l->l_type != lt_library
+	      && GL(dl_tls_dtv_slotinfo_list) == NULL)
+	    {
 #ifdef SHARED
 	      /* We are loading the executable itself when the dynamic
 		 linker was executed directly.  The setup will happen
-	     later.  Otherwise, the TLS data structures are already
+		 later.  */
-	     initialized, and we assigned a TLS modid above.  */
 	      assert (l->l_prev == NULL || (mode & __RTLD_AUDIT) != 0);
 #else
 	      assert (false && "TLS not initialized in static application");
 #endif
+	    }
 	  break;
 
 	case PT_GNU_STACK:
@@ -1379,6 +1373,18 @@ cannot enable executable stack as shared object requires");
     add_name_to_object (l, ((const char *) D_PTR (l, l_info[DT_STRTAB])
 			    + l->l_info[DT_SONAME]->d_un.d_val));
 
+  /* _dl_close can only eventually undo the module ID assignment (via
+     remove_slotinfo) if this function returns a pointer to a link
+     map.  Therefore, delay this step until all possibilities for
+     failure have been excluded.  */
+  if (l->l_tls_blocksize > 0
+      && (__glibc_likely (l->l_type == lt_library)
+	  /* If GL(dl_tls_dtv_slotinfo_list) == NULL, then rtld.c did
+	     not set up TLS data structures, so don't use them now.  */
+	  || __glibc_likely (GL(dl_tls_dtv_slotinfo_list) != NULL)))
+    /* Assign the next available module ID.  */
+    l->l_tls_modid = _dl_next_tls_modid ();
+
 #ifdef DL_AFTER_LOAD
   DL_AFTER_LOAD (l);
 #endif
@@ -1646,17 +1652,6 @@ open_verify (const char *name, int fd,
 	  errstring = N_("only ET_DYN and ET_EXEC can be loaded");
 	  goto call_lose;
 	}
-      else if (__glibc_unlikely (ehdr->e_type == ET_EXEC
-				 && (mode & __RTLD_OPENEXEC) == 0))
-	{
-	  /* BZ #16634. It is an error to dlopen ET_EXEC (unless
-	     __RTLD_OPENEXEC is explicitly set).  We return error here
-	     so that code in _dl_map_object_from_fd does not try to set
-	     l_tls_modid for this module.  */
-
-	  errstring = N_("cannot dynamically load executable");
-	  goto call_lose;
-	}
       else if (__glibc_unlikely (ehdr->e_phentsize != sizeof (ElfW(Phdr))))
 	{
 	  errstring = N_("ELF file's phentsize not the expected size");

elf/tst-dlopen-aout-pie.c

@@ -0,0 +1,19 @@
+/* Test case for BZ #16634 and BZ#24900.  PIE version.
+   Copyright (C) 2014-2019 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 "tst-dlopen-aout.c"