Subject: install-sh: avoid (low risk) race in /tmp
From: Pavel Raiskup <[email-masked]>
Date: Sat, 15 Aug 2015 04:40:57 -0400

Ensure that nobody can cross privilege boundaries by pre-creating
symlink on '$tmpdir' path.

Just testing 'mkdir -p' by creating '/tmp/ins$RANDOM-$$/d' is not
safe because '/tmp' directory is usually world-writeable and
'/tmp/ins$RANDOM-$$' content could be pretty easily guessed by
attacker (at least for shells where $RANDOM is not supported).
So, as the first step, create the '/tmp/ins$RANDOM-$$' without -p.
This step would fail early if somebody wanted catch us.

Note that systems that implement (and have enabled)
fs.protected_symlinks kernel feature are not affected even without
this commit.

References:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=760455
https://bugzilla.redhat.com/show_bug.cgi?id=1140725

* lib/install-sh: Implement safer 'mkdir -p' test by running
'$mkdirprog $mkdir_mode "$tmpdir"' first.
(scriptversion): Bump.
---
 lib/install-sh | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/lib/install-sh b/lib/install-sh
index 0b0fdcb..59990a1 100755
--- a/lib/install-sh
+++ b/lib/install-sh
@@ -324,34 +324,41 @@ do
             # is incompatible with FreeBSD 'install' when (umask & 300) != 0.
             ;;
           *)
+            # $RANDOM is not portable (e.g. dash);  use it when possible to
+            # lower collision chance
             tmpdir=${TMPDIR-/tmp}/ins$RANDOM-$$
-            trap 'ret=$?; rmdir "$tmpdir/d" "$tmpdir" 2>/dev/null; exit $ret' 0
+            trap 'ret=$?; rmdir "$tmpdir/a/b" "$tmpdir/a" "$tmpdir" 2>/dev/null; exit $ret' 0
 
+            # As "mkdir -p" follows symlinks and we work in /tmp possibly;  so
+            # create the $tmpdir first (and fail if unsuccessful) to make sure
+            # that nobody tries to guess the $tmpdir name.
             if (umask $mkdir_umask &&
-                exec $mkdirprog $mkdir_mode -p -- "$tmpdir/d") >/dev/null 2>&1
+                $mkdirprog $mkdir_mode "$tmpdir" &&
+                exec $mkdirprog $mkdir_mode -p -- "$tmpdir/a/b") >/dev/null 2>&1
             then
               if test -z "$dir_arg" || {
                    # Check for POSIX incompatibilities with -m.
                    # HP-UX 11.23 and IRIX 6.5 mkdir -m -p sets group- or
                    # other-writable bit of parent directory when it shouldn't.
                    # FreeBSD 6.1 mkdir -m -p sets mode of existing directory.
-                   ls_ld_tmpdir=`ls -ld "$tmpdir"`
+                   test_tmpdir="$tmpdir/a"
+                   ls_ld_tmpdir=`ls -ld "$test_tmpdir"`
                    case $ls_ld_tmpdir in
                      d????-?r-*) different_mode=700;;
                      d????-?--*) different_mode=755;;
                      *) false;;
                    esac &&
-                   $mkdirprog -m$different_mode -p -- "$tmpdir" && {
-                     ls_ld_tmpdir_1=`ls -ld "$tmpdir"`
+                   $mkdirprog -m$different_mode -p -- "$test_tmpdir" && {
+                     ls_ld_tmpdir_1=`ls -ld "$test_tmpdir"`
                      test "$ls_ld_tmpdir" = "$ls_ld_tmpdir_1"
                    }
                  }
               then posix_mkdir=:
               fi
-              rmdir "$tmpdir/d" "$tmpdir"
+              rmdir "$tmpdir/a/b" "$tmpdir/a" "$tmpdir"
             else
               # Remove any dirs left behind by ancient mkdir implementations.
-              rmdir ./$mkdir_mode ./-p ./-- 2>/dev/null
+              rmdir ./$mkdir_mode ./-p ./-- "$tmpdir" 2>/dev/null
             fi
             trap '' 0;;
         esac;;