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[PATCH] ld.so: Examine GLRO to detect inactive loader [BZ #20204]


GLRO (_rtld_global_ro) is read-only after initialization and can
therefore not be patched at run time, unlike the hook table addresses
and their contents, so this is a desirable hardening feature.

The hooks are only needed if ld.so has not been initialized, and this
happens only after static dlopen (dlmopen uses a single ld.so object
across all namespaces).

2017-12-18  Florian Weimer  <fweimer@redhat.com>

	[BZ #20204]
	ld.so: Harden dl-libc/libdl hooks.
	* sysdeps/generic/ldsodefs.h (rtld_active): New function.
	* dlfcn/dladdr.c (__dladdr): Call it.
	* dlfcn/dladdr1.c (__dladdr1): Likewise.
	* dlfcn/dlclose.c (__dlcose): Likewise.
	* dlfcn/dlerror.c (__dlerror): Likewise.
	* dlfcn/dlinfo.c (__dlinfo): Likewise.
	* dlfcn/dlmopen.c (__dlmopen): Likewise.
	* dlfcn/dlopen.c (__dlopen): Likewise.
	* dlfcn/dlopenold.c (__dlopen_nocheck): Likewise.
	* dlfcn/dlsym.c (__dlsym): Likewise.
	* dlfcn/dlvsym.c (__dlvsym): Likewise.
	* elf/dl-libc.c (__libc_dlopen_mode, __libc_dlsym)
	(__libc_dlclose): Likewise.

diff --git a/dlfcn/dladdr.c b/dlfcn/dladdr.c
index 1753434160..1bebd00240 100644
--- a/dlfcn/dladdr.c
+++ b/dlfcn/dladdr.c
@@ -17,6 +17,7 @@
    <http://www.gnu.org/licenses/>.  */
 
 #include <dlfcn.h>
+#include <ldsodefs.h>
 
 #if !defined SHARED && IS_IN (libdl)
 
@@ -32,7 +33,7 @@ int
 __dladdr (const void *address, Dl_info *info)
 {
 # ifdef SHARED
-  if (__glibc_unlikely (_dlfcn_hook != NULL))
+  if (!rtld_active ())
     return _dlfcn_hook->dladdr (address, info);
 # endif
   return _dl_addr (address, info, NULL, NULL);
diff --git a/dlfcn/dladdr1.c b/dlfcn/dladdr1.c
index a19f9fdea2..901cf43f6b 100644
--- a/dlfcn/dladdr1.c
+++ b/dlfcn/dladdr1.c
@@ -17,6 +17,7 @@
    <http://www.gnu.org/licenses/>.  */
 
 #include <dlfcn.h>
+#include <ldsodefs.h>
 
 #if !defined SHARED && IS_IN (libdl)
 
@@ -32,7 +33,7 @@ int
 __dladdr1 (const void *address, Dl_info *info, void **extra, int flags)
 {
 # ifdef SHARED
-  if (__glibc_unlikely (_dlfcn_hook != NULL))
+  if (!rtld_active ())
     return _dlfcn_hook->dladdr1 (address, info, extra, flags);
 # endif
 
diff --git a/dlfcn/dlclose.c b/dlfcn/dlclose.c
index da66e20488..223887d338 100644
--- a/dlfcn/dlclose.c
+++ b/dlfcn/dlclose.c
@@ -39,7 +39,7 @@ int
 __dlclose (void *handle)
 {
 # ifdef SHARED
-  if (__glibc_unlikely (_dlfcn_hook != NULL))
+  if (!rtld_active ())
     return _dlfcn_hook->dlclose (handle);
 # endif
 
diff --git a/dlfcn/dlerror.c b/dlfcn/dlerror.c
index fb5012ee85..b33c05095a 100644
--- a/dlfcn/dlerror.c
+++ b/dlfcn/dlerror.c
@@ -63,7 +63,7 @@ __dlerror (void)
   struct dl_action_result *result;
 
 # ifdef SHARED
-  if (__glibc_unlikely (_dlfcn_hook != NULL))
+  if (!rtld_active ())
     return _dlfcn_hook->dlerror ();
 # endif
 
diff --git a/dlfcn/dlinfo.c b/dlfcn/dlinfo.c
index a34e947ed3..b011257468 100644
--- a/dlfcn/dlinfo.c
+++ b/dlfcn/dlinfo.c
@@ -111,7 +111,7 @@ int
 __dlinfo (void *handle, int request, void *arg DL_CALLER_DECL)
 {
 # ifdef SHARED
-  if (__glibc_unlikely (_dlfcn_hook != NULL))
+  if (!rtld_active ())
     return _dlfcn_hook->dlinfo (handle, request, arg,
 				DL_CALLER);
 # endif
diff --git a/dlfcn/dlmopen.c b/dlfcn/dlmopen.c
index 07d59ade30..58f88bb7c6 100644
--- a/dlfcn/dlmopen.c
+++ b/dlfcn/dlmopen.c
@@ -79,7 +79,7 @@ void *
 __dlmopen (Lmid_t nsid, const char *file, int mode DL_CALLER_DECL)
 {
 # ifdef SHARED
-  if (__glibc_unlikely (_dlfcn_hook != NULL))
+  if (!rtld_active ())
     return _dlfcn_hook->dlmopen (nsid, file, mode, RETURN_ADDRESS (0));
 # endif
 
diff --git a/dlfcn/dlopen.c b/dlfcn/dlopen.c
index 22120655d2..73651a8430 100644
--- a/dlfcn/dlopen.c
+++ b/dlfcn/dlopen.c
@@ -74,7 +74,7 @@ void *
 __dlopen (const char *file, int mode DL_CALLER_DECL)
 {
 # ifdef SHARED
-  if (__glibc_unlikely (_dlfcn_hook != NULL))
+  if (!rtld_active ())
     return _dlfcn_hook->dlopen (file, mode, DL_CALLER);
 # endif
 
diff --git a/dlfcn/dlopenold.c b/dlfcn/dlopenold.c
index a3db500705..d899c4e890 100644
--- a/dlfcn/dlopenold.c
+++ b/dlfcn/dlopenold.c
@@ -70,7 +70,7 @@ __dlopen_nocheck (const char *file, int mode)
     mode |= RTLD_LAZY;
   args.mode = mode;
 
-  if (__glibc_unlikely (_dlfcn_hook != NULL))
+  if (!rtld_active ())
     return _dlfcn_hook->dlopen (file, mode, RETURN_ADDRESS (0));
 
   return _dlerror_run (dlopen_doit, &args) ? NULL : args.new;
diff --git a/dlfcn/dlsym.c b/dlfcn/dlsym.c
index 7976c5f75c..19733a0f19 100644
--- a/dlfcn/dlsym.c
+++ b/dlfcn/dlsym.c
@@ -55,7 +55,7 @@ void *
 __dlsym (void *handle, const char *name DL_CALLER_DECL)
 {
 # ifdef SHARED
-  if (__glibc_unlikely (_dlfcn_hook != NULL))
+  if (!rtld_active ())
     return _dlfcn_hook->dlsym (handle, name, DL_CALLER);
 # endif
 
diff --git a/dlfcn/dlvsym.c b/dlfcn/dlvsym.c
index 5ed220b77c..ad46b65023 100644
--- a/dlfcn/dlvsym.c
+++ b/dlfcn/dlvsym.c
@@ -58,7 +58,7 @@ __dlvsym (void *handle, const char *name, const char *version_str
 	  DL_CALLER_DECL)
 {
 # ifdef SHARED
-  if (__glibc_unlikely (_dlfcn_hook != NULL))
+  if (!rtld_active ())
     return _dlfcn_hook->dlvsym (handle, name, version_str, DL_CALLER);
 # endif
 
diff --git a/elf/dl-libc.c b/elf/dl-libc.c
index bd3c18d20f..7d9a8948f3 100644
--- a/elf/dl-libc.c
+++ b/elf/dl-libc.c
@@ -157,7 +157,7 @@ __libc_dlopen_mode (const char *name, int mode)
   args.caller_dlopen = RETURN_ADDRESS (0);
 
 #ifdef SHARED
-  if (__glibc_unlikely (_dl_open_hook != NULL))
+  if (!rtld_active ())
     return _dl_open_hook->dlopen_mode (name, mode);
   return (dlerror_run (do_dlopen, &args) ? NULL : (void *) args.map);
 #else
@@ -203,7 +203,7 @@ __libc_dlsym (void *map, const char *name)
   args.name = name;
 
 #ifdef SHARED
-  if (__glibc_unlikely (_dl_open_hook != NULL))
+  if (!rtld_active ())
     return _dl_open_hook->dlsym (map, name);
 #endif
   return (dlerror_run (do_dlsym, &args) ? NULL
@@ -215,7 +215,7 @@ int
 __libc_dlclose (void *map)
 {
 #ifdef SHARED
-  if (__glibc_unlikely (_dl_open_hook != NULL))
+  if (!rtld_active ())
     return _dl_open_hook->dlclose (map);
 #endif
   return dlerror_run (do_dlclose, map);
diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index 196513851f..61313a72ba 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -1144,6 +1144,20 @@ extern void _dl_non_dynamic_init (void)
 extern void _dl_aux_init (ElfW(auxv_t) *av)
      attribute_hidden;
 
+/* Return true if the ld.so copy in this namespace is actually active
+   and working.  If false, the dl_open/dlfcn hooks have to be used to
+   call into the outer dynamic linker (which happens after static
+   dlopen).  */
+#ifdef SHARED
+static inline bool
+rtld_active (void)
+{
+  /* The default-initialized variable does not have a non-zero
+     dl_init_all_dirs member, so this allows us to recognize an
+     initialized and active ld.so copy.  */
+  return GLRO(dl_init_all_dirs) != NULL;
+}
+#endif
 
 __END_DECLS
 


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