Fix CVE-2022-4883: compression commands depend on $PATH

By default, on all platforms except MinGW, libXpm will detect if a
filename ends in .Z or .gz, and will when reading such a file fork off
an uncompress or gunzip command to read from via a pipe, and when
writing such a file will fork off a compress or gzip command to write
to via a pipe.

In libXpm 3.5.14 or older these are run via execlp(), relying on $PATH
to find the commands.  If libXpm is called from a program running with
raised privileges, such as via setuid, then a malicious user could set
$PATH to include programs of their choosing to be run with those
privileges.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>

README.md

@@ -31,3 +31,15 @@ if it can't find the file it was asked to open.  It relies on the
 --enable-open-zfile feature to open the file, and is enabled by default
 when --enable-open-zfile is enabled, and can be disabled by passing the
 --disable-stat-zfile flag to the configure script.
+
+All of these commands will be executed with whatever userid & privileges the
+function is called with, relying on the caller to ensure the correct euid,
+egid, etc. are set before calling.
+
+To reduce risk, the paths to these commands are now set at configure time to
+the first version found in the PATH used to run configure, and do not depend
+on the PATH environment variable set at runtime.
+
+To specify paths to be used for these commands instead of searching $PATH, pass
+the XPM_PATH_COMPRESS, XPM_PATH_UNCOMPRESS, XPM_PATH_GZIP, and XPM_PATH_GUNZIP
+variables to the configure command.

configure.ac

@@ -49,6 +49,14 @@ if test "x$USE_GETTEXT" = "xyes" ; then
 fi
 AM_CONDITIONAL(USE_GETTEXT, test "x$USE_GETTEXT" = "xyes")
 
+dnl Helper macro to find absolute path to program and add a #define for it
+AC_DEFUN([XPM_PATH_PROG],[
+AC_PATH_PROG([$1], [$2], [])
+AS_IF([test "x$$1" = "x"],
+      [AC_MSG_ERROR([$2 not found, set $1 or use --disable-stat-zfile])])
+AC_DEFINE_UNQUOTED([$1], ["$$1"], [Path to $2])
+]) dnl End of AC_DEFUN([XPM_PATH_PROG]...
+
 # Optional feature: When a filename ending in .Z or .gz is requested,
 # open a pipe to a newly forked compress/uncompress/gzip/gunzip command to
 # handle it.
@@ -65,6 +73,12 @@ AC_MSG_RESULT([$OPEN_ZFILE])
 AM_CONDITIONAL(COMPRESSED_PIXMAPS, test "x$OPEN_ZFILE" = "xyes")
 if test x$OPEN_ZFILE = xno ; then
         AC_DEFINE(NO_ZPIPE, 1, [Define to 1 to disable decompression via pipes])
+else
+        XPM_PATH_PROG([XPM_PATH_COMPRESS], [compress])
+        XPM_PATH_PROG([XPM_PATH_UNCOMPRESS], [uncompress])
+        XPM_PATH_PROG([XPM_PATH_GZIP], [gzip])
+        XPM_PATH_PROG([XPM_PATH_GUNZIP], [gunzip])
+        AC_CHECK_FUNCS([closefrom close_range], [break])
 fi
 
 # Optional feature: When ___.xpm is requested, also look for ___.xpm.Z & .gz

src/RdFToI.c

@@ -43,6 +43,7 @@
 #include <errno.h>
 #include <sys/types.h>
 #include <sys/wait.h>
+#include <unistd.h>
 #else
 #ifdef FOR_MSW
 #include <fcntl.h>
@@ -161,7 +162,17 @@ xpmPipeThrough(
 	    goto err;
 	if ( 0 == pid )
 	{
-	    execlp(cmd, cmd, arg1, (char *)NULL);
+#ifdef HAVE_CLOSEFROM
+	    closefrom(3);
+#elif defined(HAVE_CLOSE_RANGE)
+# ifdef CLOSE_RANGE_UNSHARE
+#  define close_range_flags CLOSE_RANGE_UNSHARE
+# else
+#  define close_range_flags 0
+#endif
+	    close_range(3, ~0U, close_range_flags);
+#endif
+	    execl(cmd, cmd, arg1, (char *)NULL);
 	    perror(cmd);
 	    goto err;
 	}
@@ -235,12 +246,12 @@ OpenReadFile(
 	if ( ext && !strcmp(ext, ".Z") )
 	{
 	    mdata->type = XPMPIPE;
-	    mdata->stream.file = xpmPipeThrough(fd, "uncompress", "-c", "r");
+	    mdata->stream.file = xpmPipeThrough(fd, XPM_PATH_UNCOMPRESS, "-c", "r");
 	}
 	else if ( ext && !strcmp(ext, ".gz") )
 	{
 	    mdata->type = XPMPIPE;
-	    mdata->stream.file = xpmPipeThrough(fd, "gunzip", "-qc", "r");
+	    mdata->stream.file = xpmPipeThrough(fd, XPM_PATH_GUNZIP, "-qc", "r");
 	}
 	else
 #endif /* z-files */

src/WrFFrI.c

@@ -342,10 +342,10 @@ OpenWriteFile(
 #ifndef NO_ZPIPE
 	len = strlen(filename);
 	if (len > 2 && !strcmp(".Z", filename + (len - 2))) {
-	    mdata->stream.file = xpmPipeThrough(fd, "compress", NULL, "w");
+	    mdata->stream.file = xpmPipeThrough(fd, XPM_PATH_COMPRESS, NULL, "w");
 	    mdata->type = XPMPIPE;
 	} else if (len > 3 && !strcmp(".gz", filename + (len - 3))) {
-	    mdata->stream.file = xpmPipeThrough(fd, "gzip", "-q", "w");
+	    mdata->stream.file = xpmPipeThrough(fd, XPM_PATH_GZIP, "-q", "w");
 	    mdata->type = XPMPIPE;
 	} else
 #endif