[PATCH 16/25] virtiofsd: sandbox mount namespace

[Dr. David Alan Gilbert (git)]

From: Stefan Hajnoczi <address@hidden>

Use a mount namespace with the shared directory tree mounted at "/" and
no other mounts.

This prevents symlink escape attacks because symlink targets are
resolved only against the shared directory and cannot go outside it.

Signed-off-by: Stefan Hajnoczi <address@hidden>
Signed-off-by: Peng Tao <address@hidden>
---
 contrib/virtiofsd/passthrough_ll.c | 89 ++++++++++++++++++++++++++++++
 1 file changed, 89 insertions(+)

diff --git a/contrib/virtiofsd/passthrough_ll.c 
b/contrib/virtiofsd/passthrough_ll.c
index 3ddf22d162..20a34d4d83 100644
--- a/contrib/virtiofsd/passthrough_ll.c
+++ b/contrib/virtiofsd/passthrough_ll.c
@@ -51,6 +51,7 @@
 #include <sys/file.h>
 #include <sys/syscall.h>
 #include <sys/xattr.h>
+#include <sys/mount.h>
 
 #include "passthrough_helpers.h"
 
@@ -1821,6 +1822,58 @@ static void print_capabilities(void)
        printf("}\n");
 }
 
+/* This magic is based on lxc's lxc_pivot_root() */
+static void setup_pivot_root(const char *source)
+{
+       int oldroot;
+       int newroot;
+
+       oldroot = open("/", O_DIRECTORY | O_RDONLY | O_CLOEXEC);
+       if (oldroot < 0) {
+               fuse_log(FUSE_LOG_ERR, "open(/): %m\n");
+               exit(1);
+       }
+
+       newroot = open(source, O_DIRECTORY | O_RDONLY | O_CLOEXEC);
+       if (newroot < 0) {
+               fuse_log(FUSE_LOG_ERR, "open(%s): %m\n", source);
+               exit(1);
+       }
+
+       if (fchdir(newroot) < 0) {
+               fuse_log(FUSE_LOG_ERR, "fchdir(newroot): %m\n");
+               exit(1);
+       }
+
+       if (syscall(__NR_pivot_root, ".", ".") < 0){
+               fuse_log(FUSE_LOG_ERR, "pivot_root(., .): %m\n");
+               exit(1);
+       }
+
+       if (fchdir(oldroot) < 0) {
+               fuse_log(FUSE_LOG_ERR, "fchdir(oldroot): %m\n");
+               exit(1);
+       }
+
+       if (mount("", ".", "", MS_SLAVE | MS_REC, NULL) < 0) {
+               fuse_log(FUSE_LOG_ERR, "mount(., MS_SLAVE | MS_REC): %m\n");
+               exit(1);
+       }
+
+       if (umount2(".", MNT_DETACH) < 0) {
+               fuse_log(FUSE_LOG_ERR, "umount2(., MNT_DETACH): %m\n");
+               exit(1);
+       }
+
+       if (fchdir(newroot) < 0) {
+               fuse_log(FUSE_LOG_ERR, "fchdir(newroot): %m\n");
+               exit(1);
+       }
+
+       close(newroot);
+       close(oldroot);
+}
+
 static void setup_proc_self_fd(struct lo_data *lo)
 {
        lo->proc_self_fd = open("/proc/self/fd", O_PATH);
@@ -1830,6 +1883,39 @@ static void setup_proc_self_fd(struct lo_data *lo)
        }
 }
 
+/*
+ * Make the source directory our root so symlinks cannot escape and no other
+ * files are accessible.
+ */
+static void setup_mount_namespace(const char *source)
+{
+       if (unshare(CLONE_NEWNS) != 0) {
+               fuse_log(FUSE_LOG_ERR, "unshare(CLONE_NEWNS): %m\n");
+               exit(1);
+       }
+
+       if (mount(NULL, "/", NULL, MS_REC|MS_SLAVE, NULL) < 0) {
+               fuse_log(FUSE_LOG_ERR, "mount(/, MS_REC|MS_PRIVATE): %m\n");
+               exit(1);
+       }
+
+       if (mount(source, source, NULL, MS_BIND, NULL) < 0) {
+               fuse_log(FUSE_LOG_ERR, "mount(%s, %s, MS_BIND): %m\n", source, 
source);
+               exit(1);
+       }
+
+       setup_pivot_root(source);
+}
+
+/*
+ * Lock down this process to prevent access to other processes or files outside
+ * source directory.  This reduces the impact of arbitrary code execution bugs.
+ */
+static void setup_sandbox(struct lo_data *lo)
+{
+       setup_mount_namespace(lo->source);
+}
+
 int main(int argc, char *argv[])
 {
        struct fuse_args args = FUSE_ARGS_INIT(argc, argv);
@@ -1927,6 +2013,7 @@ int main(int argc, char *argv[])
        }
 
        lo.root.fd = open(lo.source, O_PATH);
+
        if (lo.root.fd == -1) {
                fuse_log(FUSE_LOG_ERR, "open(\"%s\", O_PATH): %m\n",
                         lo.source);
@@ -1948,6 +2035,8 @@ int main(int argc, char *argv[])
        /* Must be after daemonize to get the right /proc/self/fd */
        setup_proc_self_fd(&lo);
 
+       setup_sandbox(&lo);
+
        /* Block until ctrl+c or fusermount -u */
        ret = virtio_loop(se);
 
-- 
2.23.0