From: Stefan Hajnoczi <stefanha@redhat.com>
Date: Mon, 27 Jan 2020 19:01:27 +0000
Subject: [PATCH] virtiofsd: sandbox mount namespace
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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 <stefanha@redhat.com>
Signed-off-by: Peng Tao <tao.peng@linux.alibaba.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
(cherry picked from commit 5baa3b8e95064c2434bd9e2f312edd5e9ae275dc)
---
 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 e2e2211ea1..0570453eef 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -50,6 +50,7 @@
 #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>
@@ -1943,6 +1944,58 @@ 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);
@@ -1952,6 +2005,39 @@ 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);
@@ -2052,6 +2138,7 @@ 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);
@@ -2075,6 +2162,8 @@ 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);