elf: Add elf/check-wx-segment, a test for the presence of WX segments

Writable, executable segments defeat security hardening.  The
existing check for DT_TEXTREL does not catch this.

hppa and SPARC currently keep the PLT in an RWX load segment.

elf/Makefile

@@ -402,6 +402,7 @@ tests-special += $(objpfx)tst-pathopt.out $(objpfx)tst-rtld-load-self.out \
 		 $(objpfx)tst-rtld-preload.out
 endif
 tests-special += $(objpfx)check-textrel.out $(objpfx)check-execstack.out \
+		 $(objpfx)check-wx-segment.out \
 		 $(objpfx)check-localplt.out $(objpfx)check-initfini.out
 endif
 
@@ -1180,6 +1181,12 @@ $(objpfx)check-execstack.out: $(..)scripts/check-execstack.awk \
 	$(evaluate-test)
 generated += check-execstack.out
 
+$(objpfx)check-wx-segment.out: $(..)scripts/check-wx-segment.py \
+			      $(all-built-dso:=.phdr)
+	$(PYTHON) $^ --xfail="$(check-wx-segment-xfail)" > $@; \
+	$(evaluate-test)
+generated += check-wx-segment.out
+
 $(objpfx)tst-dlmodcount: $(libdl)
 $(objpfx)tst-dlmodcount.out: $(test-modules)
 

scripts/check-wx-segment.py

@@ -0,0 +1,85 @@
+#!/usr/bin/python3
+# Check ELF program headers for WX segments.
+# Copyright (C) 2020 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/>.
+
+"""Check that the program headers do not contain write-exec segments."""
+
+import argparse
+import os.path
+import re
+import sys
+
+# Regular expression to extract the RWE flags field.  The
+# address/offset columns have varying width.
+RE_LOAD = re.compile(
+    r'^  LOAD +(?:0x[0-9a-fA-F]+ +){5}([R ][W ][ E]) +0x[0-9a-fA-F]+\n\Z')
+
+def process_file(path, inp, xfail):
+    """Analyze one input file."""
+
+    errors = 0
+    for line in inp:
+        error = None
+        if line.startswith('  LOAD '):
+            match = RE_LOAD.match(line)
+            if match is None:
+                error = 'Invalid LOAD line'
+            else:
+                flags, = match.groups()
+                if 'W' in flags and 'E' in flags:
+                    if xfail:
+                        print('{}: warning: WX segment (as expected)'.format(
+                            path))
+                    else:
+                        error = 'WX segment'
+
+        if error is not None:
+            print('{}: error: {}: {!r}'.format(path, error, line.strip()))
+            errors += 1
+
+    if xfail and errors == 0:
+        print('{}: warning: missing expected WX segment'.format(path))
+    return errors
+
+
+def main():
+    """The main entry point."""
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument('--xfail',
+                        help='Mark input files as XFAILed ("*" for all)',
+                        type=str, default='')
+    parser.add_argument('phdrs',
+                        help='Files containing readelf -Wl output',
+                        nargs='*')
+    opts = parser.parse_args(sys.argv)
+
+    xfails = set(opts.xfail.split(' '))
+    xfails_all = opts.xfail.strip() == '*'
+
+    errors = 0
+    for path in opts.phdrs:
+        xfail = ((os.path.basename(path) + '.phdrs') in xfails
+                 or xfails_all)
+        with open(path) as inp:
+            errors += process_file(path, inp, xfail)
+    if errors > 0:
+        sys.exit(1)
+
+
+if __name__ == '__main__':
+    main()

sysdeps/sparc/Makefile

@@ -16,6 +16,15 @@ CPPFLAGS-crti.S += -fPIC
 CPPFLAGS-crtn.S += -fPIC
 endif
 
+ifeq ($(subdir),elf)
+
+# Lazy binding on SPARC rewrites the PLT sequence.  See the Solaris
+# Linker and Libraries Guide, section SPARC: Procedure Linkage Table.
+# <https://docs.oracle.com/cd/E19455-01/816-0559/chapter6-1236/index.html>
+test-xfail-check-wx-segment = *
+
+endif # $(subdir) == elf
+
 # nscd uses atomic_spin_nop which in turn requires cpu_relax
 ifeq ($(subdir),nscd)
 routines += cpu_relax

sysdeps/unix/sysv/linux/hppa/Makefile

@@ -3,9 +3,14 @@ ifeq ($(subdir),stdlib)
 gen-as-const-headers += ucontext_i.sym
 endif
 
+ifeq ($(subdir),elf)
 # Supporting non-executable stacks on HPPA requires changes to both
 # the Linux kernel and glibc. The kernel currently needs an executable
 # stack for syscall restarts and signal returns.
-ifeq ($(subdir),elf)
 test-xfail-check-execstack = yes
-endif
+
+# On hppa, the PLT is executable because it contains an executable
+# trampoline used during lazy binding.
+test-xfail-check-wx-segment = *
+
+endif # $(subdir) == elf