[PATCH 2/2] Add run-time chesk for indirect external access

["H.J. Lu via Libc-alpha" <libc-alpha@sourceware.org>]

When performing symbol lookup for references in executable without
indirect external access:

1. Disallow copy relocations in executable against protected data symbols
in a shared object with indirect external access.
2. Disallow non-zero symbol values of undefined function symbols in
executable, which are used as the function pointer, against protected
function symbols in a shared object with indirect external access.
---
 elf/dl-lookup.c                |  5 ++++
 sysdeps/generic/dl-protected.h | 54 ++++++++++++++++++++++++++++++++++
 2 files changed, 59 insertions(+)
 create mode 100644 sysdeps/generic/dl-protected.h

diff --git a/elf/dl-lookup.c b/elf/dl-lookup.c
index eea217eb28..430359af39 100644
--- a/elf/dl-lookup.c
+++ b/elf/dl-lookup.c
@@ -24,6 +24,7 @@
 #include <ldsodefs.h>
 #include <dl-hash.h>
 #include <dl-machine.h>
+#include <dl-protected.h>
 #include <sysdep-cancel.h>
 #include <libc-lock.h>
 #include <tls.h>
@@ -527,6 +528,10 @@ do_lookup_x (const char *undef_name, uint_fast32_t new_hash,
 	  if (__glibc_unlikely (dl_symbol_visibility_binds_local_p (sym)))
 	    goto skip;
 
+	  if (ELFW(ST_VISIBILITY) (sym->st_other) == STV_PROTECTED)
+	    _dl_check_protected_symbol (undef_name, undef_map, ref, map,
+					type_class);
+
 	  switch (ELFW(ST_BIND) (sym->st_info))
 	    {
 	    case STB_WEAK:
diff --git a/sysdeps/generic/dl-protected.h b/sysdeps/generic/dl-protected.h
new file mode 100644
index 0000000000..244d020dc4
--- /dev/null
+++ b/sysdeps/generic/dl-protected.h
@@ -0,0 +1,54 @@
+/* Support for STV_PROTECTED visibility.  Generic version.
+   Copyright (C) 2021 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
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _DL_PROTECTED_H
+#define _DL_PROTECTED_H
+
+static inline void __attribute__ ((always_inline))
+_dl_check_protected_symbol (const char *undef_name,
+			    const struct link_map *undef_map,
+			    const ElfW(Sym) *ref,
+			    const struct link_map *map,
+			    int type_class)
+{
+  if (undef_map != NULL
+      && undef_map->l_type == lt_executable
+      && !(undef_map->l_1_needed
+	   & GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS)
+      && (map->l_1_needed
+	  & GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS))
+    {
+      if ((type_class & ELF_RTYPE_CLASS_COPY))
+	/* Disallow copy relocations in executable against protected
+	   data symbols in a shared object which needs indirect external
+	   access.  */
+	_dl_signal_error (0, map->l_name, undef_name,
+			  N_("copy relocation against non-copyable protected symbol"));
+      else if (ref->st_value != 0
+	       && ref->st_shndx == SHN_UNDEF
+	       && (type_class & ELF_RTYPE_CLASS_PLT))
+	/* Disallow non-zero symbol values of undefined symbols in
+	   executable, which are used as the function pointer, against
+	   protected function symbols in a shared object with indirect
+	   external access.  */
+	_dl_signal_error (0, map->l_name, undef_name,
+			  N_("non-canonical reference to canonical protected function"));
+    }
+}
+
+#endif /* _DL_PROTECTED_H */
-- 
2.31.1