From: Miklos Szeredi <mszeredi@redhat.com>

While it's not possible to escape the proc filesystem through
lo->proc_self_fd, it is possible to escape to the root of the proc
filesystem itself through "../..".

Use a temporary mount for opening lo->proc_self_fd, that has it's root at
/proc/self/fd/, preventing access to the ancestor directories.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Message-Id: <20200429124733.22488-1-mszeredi@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 tools/virtiofsd/passthrough_ll.c | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index f7b9c1d20c..d7a6474b6e 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -2536,6 +2536,8 @@ static void print_capabilities(void)
 static void setup_namespaces(struct lo_data *lo, struct fuse_session *se)
 {
     pid_t child;
+    char template[] = "virtiofsd-XXXXXX";
+    char *tmpdir;
 
     /*
      * Create a new pid namespace for *child* processes.  We'll have to
@@ -2597,12 +2599,33 @@ static void setup_namespaces(struct lo_data *lo, struct fuse_session *se)
         exit(1);
     }
 
+    tmpdir = mkdtemp(template);
+    if (!tmpdir) {
+        fuse_log(FUSE_LOG_ERR, "tmpdir(%s): %m\n", template);
+        exit(1);
+    }
+
+    if (mount("/proc/self/fd", tmpdir, NULL, MS_BIND, NULL) < 0) {
+        fuse_log(FUSE_LOG_ERR, "mount(/proc/self/fd, %s, MS_BIND): %m\n",
+                 tmpdir);
+        exit(1);
+    }
+
     /* Now we can get our /proc/self/fd directory file descriptor */
-    lo->proc_self_fd = open("/proc/self/fd", O_PATH);
+    lo->proc_self_fd = open(tmpdir, O_PATH);
     if (lo->proc_self_fd == -1) {
-        fuse_log(FUSE_LOG_ERR, "open(/proc/self/fd, O_PATH): %m\n");
+        fuse_log(FUSE_LOG_ERR, "open(%s, O_PATH): %m\n", tmpdir);
         exit(1);
     }
+
+    if (umount2(tmpdir, MNT_DETACH) < 0) {
+        fuse_log(FUSE_LOG_ERR, "umount2(%s, MNT_DETACH): %m\n", tmpdir);
+        exit(1);
+    }
+
+    if (rmdir(tmpdir) < 0) {
+        fuse_log(FUSE_LOG_ERR, "rmdir(%s): %m\n", tmpdir);
+    }
 }
 
 /*
-- 
2.26.2