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Binary hardening

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pyllyukko edited this page Nov 24, 2024 · 35 revisions

Hardening flags

To protect against various threats, it would be good to harden at least some binaries and libraries with various memory protections / binary hardening.

The following technologies should be considered:

  • RELRO
  • PIE
  • Stack canaries
    • -fstack-protector < -fstack-protector-strong < -fstack-protector-all
    • --param=ssp-buffer-size=4
    • Do note that when you're checking binary protections with checksec.sh against stripped bins, checksec.sh is unable to find the __stack_chk_fail with readelf -s as the symbol table section (.symtab) has been removed and it will print No canary found. You can use rabin2 -I file | grep '^canary' instead.
  • -D_FORTIFY_SOURCE=2
  • -ftrivial-auto-var-init=zero (not available in the GCC version currently packaged in Slackware 15.0)
  • -fstack-clash-protection

This can be achieved by using something like the following in SlackBuilds:

SLKCFLAGS="-O2 -fPIC -fPIE -pie -Wl,-z,relro,-z,now -fstack-protector-all --param=ssp-buffer-size=4 -D_FORTIFY_SOURCE=2"

PIE

Some packages can not be built with PIE, when they try to build shared libraries with it, and you can see the build failing for example like this:

building shared krb5support library (0.1)
set -x; objlist=`set -x && perl -p -e 'BEGIN { $SIG{__WARN__} = sub {die @_} }; $e=$ARGV; $e =~ s/OBJS\...$//; s/^/ /; s/ $//; s/ / $e/g;' OBJS.SH` && gcc -shared -fPIC -Wl,-h,libkrb5support.so.0 -Wl,--no-undefined -o libkrb5support.so.0.1 $objlist -Wl,--enable-new-dtags -Wl,-rpath -Wl,/usr/lib64 -L../../lib -lkeyutils -lresolv  -ldl  -Wl,--version-script binutils.versions 
++ set -x
++ perl -p -e 'BEGIN { $SIG{__WARN__} = sub {die @_} }; $e=$ARGV; $e =~ s/OBJS\...$//; s/^/ /; s/ $//; s/ / $e/g;' OBJS.SH
+ objlist=' threads.so init-addrinfo.so plugins.so errors.so k5buf.so gmt_mktime.so fake-addrinfo.so utf8.so utf8_conv.so zap.so path.so base64.so json.so hex.so hashtab.so bcmp.so strerror_r.so dir_filenames.so strlcpy.so'
+ gcc -shared -fPIC -Wl,-h,libkrb5support.so.0 -Wl,--no-undefined -o libkrb5support.so.0.1 threads.so init-addrinfo.so plugins.so errors.so k5buf.so gmt_mktime.so fake-addrinfo.so utf8.so utf8_conv.so zap.so path.so base64.so json.so hex.so hashtab.so bcmp.so strerror_r.so dir_filenames.so strlcpy.so -Wl,--enable-new-dtags -Wl,-rpath -Wl,/usr/lib64 -L../../lib -lkeyutils -lresolv -ldl -Wl,--version-script binutils.versions
/usr/bin/ld: utf8_conv.so: warning: relocation against `krb5int_utf8_mintab' in read-only section `.text'
/usr/bin/ld: threads.so: relocation R_X86_64_PC32 against symbol `stderr@@GLIBC_2.2.5' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: bad value
collect2: error: ld returned 1 exit status
make[2]: *** [Makefile:925: libkrb5support.so.0.1] Error 1
make[2]: Leaving directory '/tmp/krb5-1.21.3/src/util/support'
make[1]: *** [Makefile:857: all-recurse] Error 1
make[1]: Leaving directory '/tmp/krb5-1.21.3/src/util'
make: *** [Makefile:1521: all-recurse] Error 1

In these cases you need to remove the -fPIE -pie from SLKCFLAGS.

Which binaries

Here's some thoughts about which binaries should/could be hardened:

  • SSH (hardened by default)
  • Communication clients:
    • irssi (has canary, but no RELRO & PIE)
    • Pidgin
    • BitTorrent clients
    • Mail
      • mutt
      • Postfix
      • Dovecot
  • Libraries
    • OpenSSL
    • GnuTLS
    • gpgme
  • gpg
  • git
  • coreutils
  • Archiving tools:
    • tar
    • bzip2
    • rar
  • Tools that are used against binaries etc.
    • less
    • strings
    • file
    • *sum
    • hexdump
  • PDF readers
  • Traffic analyzers (tcpdump, wireshark)
  • Clamav
  • wget/curl
  • SUID binaries
    • sudo
  • Apache/PHP
  • krb5
  • OpenLDAP
  • PAM

TODO

ldd all the bins and find the most common libraries to harden.

Checking binaries

You can use checksec.sh to check binary hardenings. Do note, that the stack canary check will not work for stripped binaries, as the symbols do not exist and checksec.sh tries to look for __stack_chk_fail (in .dynstr section).

radare2

You can use radare2 to check hardenings:

$ rabin2 -I $(which ssh) | grep '^\(canary\|nx\|pic\|relro\|rpath\)\b'
canary   true
nx       true
pic      true
relro    full
rpath    NONE

Links

Secure Code Partitioning With ELF binaries, aka. SCOP:

A Secure ELF binary should have the following mitigations applied:

  • RELRO gcc -Wl,-z,relro,-z,now
  • SCOP gcc -Wl,-z,code-separation
  • PIE (Full ASLR) gcc -fPIC -pie
  • Stack Canaries gcc -fstack-protector
  • PaX mprotect(2) paxctl -M

Do not forget that statically linked executables do not officially support PIE or RELRO, but have had some solutions proposed in the paper "ASLR and RELRO protection for statically linked executables"

Signing

Signing binaries with either elfsign or bsign? Apparently bsign doesn't work against 64-bit binaries

Look into

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