[PATCH v2] 9pfs: prevent opening special files (CVE-2023-2861)

[Christian Schoenebeck]

The 9p protocol does not specifically define how server shall behave when
client tries to open a special file, however from security POV it does
make sense for 9p server to prohibit opening any special file on host side
in general. A sane Linux 9p client for instance would never attempt to
open a special file on host side, it would always handle those exclusively
on its guest side. A malicious client however could potentially escape
from the exported 9p tree by creating and opening a device file on host
side.

With QEMU this could only be exploited in the following unsafe setups:

  - Running QEMU binary as root AND 9p 'local' fs driver AND 'passthrough'
    security model.

or

  - Using 9p 'proxy' fs driver (which is running its helper daemon as
    root).

These setups were already discouraged for safety reasons before,
however for obvious reasons we are now tightening behaviour on this.

Fixes: CVE-2023-2861
Reported-by: Yanwu Shen <ywsPlz@gmail.com>
Reported-by: Jietao Xiao <shawtao1125@gmail.com>
Reported-by: Jinku Li <jkli@xidian.edu.cn>
Reported-by: Wenbo Shen <shenwenbo@zju.edu.cn>
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
---
 v1 -> v2:
 - Add equivalent fix for 'proxy' fs driver.
 - Minor adjustments on commit log.

 fsdev/virtfs-proxy-helper.c | 48 +++++++++++++++++++++++++++++++++++--
 hw/9pfs/9p-util.h           | 29 ++++++++++++++++++++++
 2 files changed, 75 insertions(+), 2 deletions(-)

diff --git a/fsdev/virtfs-proxy-helper.c b/fsdev/virtfs-proxy-helper.c
index 5cafcd7703..f311519fa3 100644
--- a/fsdev/virtfs-proxy-helper.c
+++ b/fsdev/virtfs-proxy-helper.c
@@ -26,6 +26,7 @@
 #include "qemu/xattr.h"
 #include "9p-iov-marshal.h"
 #include "hw/9pfs/9p-proxy.h"
+#include "hw/9pfs/9p-util.h"
 #include "fsdev/9p-iov-marshal.h"
 
 #define PROGNAME "virtfs-proxy-helper"
@@ -338,6 +339,49 @@ static void resetugid(int suid, int sgid)
     }
 }
 
+/*
+ * Open regular file or directory. Attempts to open any special file are
+ * rejected.
+ *
+ * returns file descriptor or -1 on error
+ */
+static int open_regular(const char *pathname, int flags, mode_t mode) {
+    int fd;
+    struct stat stbuf;
+
+    fd = open(pathname, flags, mode);
+    if (fd < 0) {
+        return fd;
+    }
+
+    /* CVE-2023-2861: Prohibit opening any special file directly on host
+     * (especially device files), as a compromised client could potentially
+     * gain access outside exported tree under certain, unsafe setups. We
+     * expect client to handle I/O on special files exclusively on guest side.
+     */
+    if (qemu_fstat(fd, &stbuf) < 0) {
+        close_preserve_errno(fd);
+        return -1;
+    }
+    if (!S_ISREG(stbuf.st_mode) && !S_ISDIR(stbuf.st_mode)) {
+        /* Tcreate and Tlcreate 9p messages mandate to immediately open the
+         * created file for I/O. So this is not (necessarily) due to a broken
+         * client, and hence no error message is to be reported in this case.
+         */
+        if (!(flags & O_CREAT)) {
+            error_report_once(
+                "9p: broken or compromised client detected; attempt to open "
+                "special file (i.e. neither regular file, nor directory)"
+            );
+        }
+        close(fd);
+        errno = ENXIO;
+        return -1;
+    }
+
+    return fd;
+}
+
 /*
  * send response in two parts
  * 1) ProxyHeader
@@ -682,7 +726,7 @@ static int do_create(struct iovec *iovec)
     if (ret < 0) {
         goto unmarshal_err_out;
     }
-    ret = open(path.data, flags, mode);
+    ret = open_regular(path.data, flags, mode);
     if (ret < 0) {
         ret = -errno;
     }
@@ -707,7 +751,7 @@ static int do_open(struct iovec *iovec)
     if (ret < 0) {
         goto err_out;
     }
-    ret = open(path.data, flags);
+    ret = open_regular(path.data, flags, 0);
     if (ret < 0) {
         ret = -errno;
     }
diff --git a/hw/9pfs/9p-util.h b/hw/9pfs/9p-util.h
index c314cf381d..9da1a0538d 100644
--- a/hw/9pfs/9p-util.h
+++ b/hw/9pfs/9p-util.h
@@ -13,6 +13,8 @@
 #ifndef QEMU_9P_UTIL_H
 #define QEMU_9P_UTIL_H
 
+#include "qemu/error-report.h"
+
 #ifdef O_PATH
 #define O_PATH_9P_UTIL O_PATH
 #else
@@ -95,6 +97,7 @@ static inline int errno_to_dotl(int err) {
 #endif
 
 #define qemu_openat     openat
+#define qemu_fstat      fstat
 #define qemu_fstatat    fstatat
 #define qemu_mkdirat    mkdirat
 #define qemu_renameat   renameat
@@ -118,6 +121,7 @@ static inline int openat_file(int dirfd, const char *name, 
int flags,
                               mode_t mode)
 {
     int fd, serrno, ret;
+    struct stat stbuf;
 
 #ifndef CONFIG_DARWIN
 again:
@@ -142,6 +146,31 @@ again:
         return -1;
     }
 
+    /* CVE-2023-2861: Prohibit opening any special file directly on host
+     * (especially device files), as a compromised client could potentially
+     * gain access outside exported tree under certain, unsafe setups. We
+     * expect client to handle I/O on special files exclusively on guest side.
+     */
+    if (qemu_fstat(fd, &stbuf) < 0) {
+        close_preserve_errno(fd);
+        return -1;
+    }
+    if (!S_ISREG(stbuf.st_mode) && !S_ISDIR(stbuf.st_mode)) {
+        /* Tcreate and Tlcreate 9p messages mandate to immediately open the
+         * created file for I/O. So this is not (necessarily) due to a broken
+         * client, and hence no error message is to be reported in this case.
+         */
+        if (!(flags & O_CREAT)) {
+            error_report_once(
+                "9p: broken or compromised client detected; attempt to open "
+                "special file (i.e. neither regular file, nor directory)"
+            );
+        }
+        close(fd);
+        errno = ENXIO;
+        return -1;
+    }
+
     serrno = errno;
     /* O_NONBLOCK was only needed to open the file. Let's drop it. We don't
      * do that with O_PATH since fcntl(F_SETFL) isn't supported, and openat()
-- 
2.30.2