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> --- 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 006908f25a..606824f002 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> @@ -1925,6 +1926,58 @@ 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); @@ -1934,6 +1987,39 @@ 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); @@ -2034,6 +2120,7 @@ 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); @@ -2057,6 +2144,8 @@ 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