Add and enable libseccomp support (Paul Moore)

6 files changed, 282 insertions, 3 deletions

diff --git a/ChangeLog b/ChangeLog
index 2063a23b..f76093c1 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2017-09-24  12:02  Christos Zoulas <christos@zoulas.com>
+
+	* seccomp support (Paul Moore)
+
 2017-09-02  11:53  Christos Zoulas <christos@zoulas.com>
 
 	* release 5.32
diff --git a/configure.ac b/configure.ac
index 946198be..fee4eded 100644
--- a/configure.ac
+++ b/configure.ac
@@ -39,6 +39,11 @@ AC_ARG_ENABLE(zlib,
 [AS_HELP_STRING([--disable-zlib], [disable zlib compression support @<:@default=auto@:>@])])
 AC_MSG_RESULT($enable_zlib)
 
+AC_MSG_CHECKING(for libseccomp support)
+AC_ARG_ENABLE(libseccomp,
+[AS_HELP_STRING([--disable-libseccomp], [disable libseccomp sandboxing @<:@default=auto@:>@])])
+AC_MSG_RESULT($enable_libseccomp)
+
 AC_MSG_CHECKING(for file formats in man section 5)
 AC_ARG_ENABLE(fsect-man5,
 [  --enable-fsect-man5      enable file formats in man section 5],
@@ -158,6 +163,9 @@ dnl Checks for libraries
 if test "$enable_zlib" != "no"; then
   AC_CHECK_LIB(z, gzopen)
 fi
+if test "$enable_libseccomp" != "no"; then
+    AC_CHECK_LIB(seccomp, seccomp_init)
+fi
 if test "$MINGW" = 1; then
   AC_CHECK_LIB(gnurx,regexec,,AC_MSG_ERROR([libgnurx is required to build file(1) with MinGW]))
 fi
diff --git a/src/Makefile.am b/src/Makefile.am
index 155aec44..cbdccac2 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -19,7 +19,7 @@ MINGWLIBS =
 endif
 libmagic_la_LIBADD = $(LTLIBOBJS) $(MINGWLIBS)
 
-file_SOURCES = file.c
+file_SOURCES = file.c seccomp.c
 file_LDADD = libmagic.la
 CLEANFILES = magic.h
 EXTRA_DIST = magic.h.in
diff --git a/src/file.c b/src/file.c
index 90e0b7e7..5152f423 100644
--- a/src/file.c
+++ b/src/file.c
@@ -32,7 +32,7 @@
 #include "file.h"
 
 #ifndef	lint
-FILE_RCSID("@(#)$File: file.c,v 1.173 2017/09/18 20:40:10 christos Exp $")
+FILE_RCSID("@(#)$File: file.c,v 1.174 2017/09/24 16:04:56 christos Exp $")
 #endif	/* lint */
 
 #include "magic.h"
@@ -186,6 +186,15 @@ main(int argc, char *argv[])
 
 	file_setprogname(progname);
 
+#ifdef HAVE_LIBSECCOMP
+#if 0
+	if (enable_sandbox_basic() == -1)
+#else
+	if (enable_sandbox_full() == -1)
+#endif
+		file_err(EXIT_FAILURE, "SECCOMP initialisation failed");
+#endif /* HAVE_LIBSECCOMP */
+
 #ifdef S_IFLNK
 	posixly = getenv("POSIXLY_CORRECT") != NULL;
 	flags |=  posixly ? MAGIC_SYMLINK : 0;
diff --git a/src/file.h b/src/file.h
index 3a993a3e..79490508 100644
--- a/src/file.h
+++ b/src/file.h
@@ -27,7 +27,7 @@
  */
 /*
  * file.h - definitions for file(1) program
- * @(#)$File: file.h,v 1.184 2017/09/18 20:40:10 christos Exp $
+ * @(#)$File: file.h,v 1.185 2017/09/24 16:04:56 christos Exp $
  */
 
 #ifndef __file_h__
@@ -590,6 +590,18 @@ const char *fmtcheck(const char *, const char *)
      __attribute__((__format_arg__(2)));
 #endif
 
+#ifdef HAVE_LIBSECCOMP
+// basic filter 
+// this mode should not interfere with normal operations
+// only some dangerous syscalls are blacklisted
+int enable_sandbox_basic(void);
+
+// enhanced filter 
+// this mode allows only the necessary syscalls used during normal operation
+// extensive testing required !!!
+int enable_sandbox_full(void);
+#endif
+
 protected const char *file_getprogname(void);
 protected void file_setprogname(const char *);
 protected void file_err(int, const char *, ...)
diff --git a/src/seccomp.c b/src/seccomp.c
new file mode 100644
index 00000000..88c73810
--- /dev/null
+++ b/src/seccomp.c
@@ -0,0 +1,246 @@
+/*
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice immediately at the beginning of the file, without modification,
+ *    this list of conditions, and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR
+ * ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+/*
+ * libseccomp hooks.
+ */
+#include "file.h"
+
+#ifndef	lint
+FILE_RCSID("@(#)$File: seccomp.c,v 1.1 2017/09/24 16:04:56 christos Exp $")
+#endif	/* lint */
+
+#if HAVE_LIBSECCOMP
+#include <seccomp.h> /* libseccomp */
+#include <sys/prctl.h> /* prctl */
+#include <sys/socket.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <errno.h>
+
+#define DENY_RULE(call) \
+    do \
+	if (seccomp_rule_add (ctx, SCMP_ACT_KILL, SCMP_SYS(call), 0) == -1) \
+	    goto out; \
+    while (/*CONSTCOND*/0)
+#define ALLOW_RULE(call) \
+    do \
+	if (seccomp_rule_add (ctx, SCMP_ACT_ALLOW, SCMP_SYS(call), 0) == -1) \
+	    goto out; \
+    while (/*CONSTCOND*/0)
+
+static scmp_filter_ctx ctx;
+
+
+int
+enable_sandbox_basic(void)
+{
+
+	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) == -1)
+		return -1;
+
+#if 0
+	// prevent escape via ptrace
+	prctl(PR_SET_DUMPABLE, 0);
+#endif
+
+	if (prctl (PR_SET_DUMPABLE, 0, 0, 0, 0) == -1)
+		return -1;
+
+	// initialize the filter
+	ctx = seccomp_init(SCMP_ACT_ALLOW);
+	if (ctx == NULL)
+	    return 1;
+
+	DENY_RULE(_sysctl);
+	DENY_RULE(acct);
+	DENY_RULE(add_key);
+	DENY_RULE(adjtimex);
+	DENY_RULE(chroot);
+	DENY_RULE(clock_adjtime);
+	DENY_RULE(create_module);
+	DENY_RULE(delete_module);
+	DENY_RULE(fanotify_init);
+	DENY_RULE(finit_module);
+	DENY_RULE(get_kernel_syms);
+	DENY_RULE(get_mempolicy);
+	DENY_RULE(init_module);
+	DENY_RULE(io_cancel);
+	DENY_RULE(io_destroy);
+	DENY_RULE(io_getevents);
+	DENY_RULE(io_setup);
+	DENY_RULE(io_submit);
+	DENY_RULE(ioperm);
+	DENY_RULE(iopl);
+	DENY_RULE(ioprio_set);
+	DENY_RULE(kcmp);
+#ifdef __NR_kexec_file_load
+	DENY_RULE(kexec_file_load);
+#endif
+	DENY_RULE(kexec_load);
+	DENY_RULE(keyctl);
+	DENY_RULE(lookup_dcookie);
+	DENY_RULE(mbind);
+	DENY_RULE(nfsservctl);
+	DENY_RULE(migrate_pages);
+	DENY_RULE(modify_ldt);
+	DENY_RULE(mount);
+	DENY_RULE(move_pages);
+	DENY_RULE(name_to_handle_at);
+	DENY_RULE(open_by_handle_at);
+	DENY_RULE(perf_event_open);
+	DENY_RULE(pivot_root);
+	DENY_RULE(process_vm_readv);
+	DENY_RULE(process_vm_writev);
+	DENY_RULE(ptrace);
+	DENY_RULE(reboot);
+	DENY_RULE(remap_file_pages);
+	DENY_RULE(request_key);
+	DENY_RULE(set_mempolicy);
+	DENY_RULE(swapoff);
+	DENY_RULE(swapon);
+	DENY_RULE(sysfs);
+	DENY_RULE(syslog);
+	DENY_RULE(tuxcall);
+	DENY_RULE(umount2);
+	DENY_RULE(uselib);
+	DENY_RULE(vmsplice);
+
+	// blocking dangerous syscalls that file should not need
+	DENY_RULE (execve);
+	DENY_RULE (socket);
+	// ...
+
+	
+	// applying filter...
+	if (seccomp_load (ctx) == -1)
+		goto out;
+	// free ctx after the filter has been loaded into the kernel
+	seccomp_release(ctx);
+	return 0;
+	
+out:
+	seccomp_release(ctx);
+	return -1;
+}
+
+
+int
+enable_sandbox_full(void)
+{
+
+	// prevent child processes from getting more priv e.g. via setuid,
+	// capabilities, ...
+	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) == -1)
+		return -1;
+
+	if (prctl(PR_SET_DUMPABLE, 0, 0, 0, 0) == -1)
+		return -1;
+	
+	// initialize the filter
+	ctx = seccomp_init(SCMP_ACT_KILL);
+	if (ctx == NULL)
+		return -1;
+
+	ALLOW_RULE(access);
+	ALLOW_RULE(brk);
+	ALLOW_RULE(close);
+	ALLOW_RULE(dup2);
+	ALLOW_RULE(exit);
+	ALLOW_RULE(exit_group);
+	ALLOW_RULE(fcntl);  
+	ALLOW_RULE(fstat);
+	ALLOW_RULE(getdents);
+	ALLOW_RULE(ioctl);
+	ALLOW_RULE(lseek);
+	ALLOW_RULE(lstat);
+	ALLOW_RULE(mmap);
+	ALLOW_RULE(mprotect);
+	ALLOW_RULE(mremap);
+	ALLOW_RULE(munmap);
+	ALLOW_RULE(open);
+	ALLOW_RULE(openat);
+	ALLOW_RULE(pread64);
+	ALLOW_RULE(read);
+	ALLOW_RULE(rt_sigaction);
+	ALLOW_RULE(rt_sigprocmask);
+	ALLOW_RULE(rt_sigreturn);
+	ALLOW_RULE(select);
+	ALLOW_RULE(stat);
+	ALLOW_RULE(sysinfo);
+	ALLOW_RULE(unlink);
+	ALLOW_RULE(write);
+
+
+#if 0
+	// needed by valgrind
+	ALLOW_RULE(gettid);
+	ALLOW_RULE(getpid);
+	ALLOW_RULE(readlink);
+	ALLOW_RULE(rt_sigtimedwait);
+#endif
+
+#if 0
+	 /* special restrictions for socket, only allow AF_UNIX/AF_LOCAL */
+	 if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(socket), 1,
+	     SCMP_CMP(0, SCMP_CMP_EQ, AF_UNIX)) == -1)
+	 	goto out;
+
+	 if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(socket), 1,
+	     SCMP_CMP(0, SCMP_CMP_EQ, AF_LOCAL)) == -1)
+	 	goto out;
+
+
+	 /* special restrictions for open, prevent opening files for writing */
+	 if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(open), 1,
+	     SCMP_CMP(1, SCMP_CMP_MASKED_EQ, O_WRONLY | O_RDWR, 0)) == -1)
+	 	goto out;
+
+	 if (seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EACCES), SCMP_SYS(open), 1,
+	     SCMP_CMP(1, SCMP_CMP_MASKED_EQ, O_WRONLY, O_WRONLY)) == -1)
+	 	goto out;
+
+	 if (seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EACCES), SCMP_SYS(open), 1,
+	     SCMP_CMP(1, SCMP_CMP_MASKED_EQ, O_RDWR, O_RDWR)) == -1)
+	 	goto out;
+
+
+	 /* allow stderr */
+	 if (seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(write), 1,
+	     SCMP_CMP(0, SCMP_CMP_EQ, 2)) == -1)
+		 goto out;
+#endif
+
+	// applying filter...
+	if (seccomp_load(ctx) == -1)
+		goto out;
+	// free ctx after the filter has been loaded into the kernel
+	seccomp_release(ctx);
+	return 0;
+
+out:
+	// something went wrong
+	seccomp_release(ctx);
+	return -1;
+}
+#endif