From: Stefan Hajnoczi <stefa...@redhat.com>
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 <stefa...@redhat.com>
Signed-off-by: Peng Tao <tao.p...@linux.alibaba.com>
Reviewed-by: Daniel P. Berrangé <berra...@redhat.com>
---
tools/virtiofsd/passthrough_ll.c | 89 ++++++++++++++++++++++++++++++++
1 file changed, 89 insertions(+)
diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index 322205ba3a..e6ecf95e31 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -49,6 +49,7 @@
#include <stdlib.h>
#include <string.h>
#include <sys/file.h>
+#include <sys/mount.h>
#include <sys/syscall.h>
#include <sys/xattr.h>
#include <unistd.h>
@@ -1942,6 +1943,58 @@ 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);
@@ -1951,6 +2004,39 @@ 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);
@@ -2051,6 +2137,7 @@ 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);
exit(1);
@@ -2074,6 +2161,8 @@ 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);
--
2.24.1
