9pfs: local: fix fchmodat_nofollow() limitations

This function has to ensure it doesn't follow a symlink that could be used
to escape the virtfs directory. This could be easily achieved if fchmodat()
on linux honored the AT_SYMLINK_NOFOLLOW flag as described in POSIX, but
it doesn't. There was a tentative to implement a new fchmodat2() syscall
with the correct semantics:

https://patchwork.kernel.org/patch/9596301/

but it didn't gain much momentum. Also it was suggested to look at an O_PATH
based solution in the first place.

The current implementation covers most use-cases, but it notably fails if:
- the target path has access rights equal to 0000 (openat() returns EPERM),
  => once you've done chmod(0000) on a file, you can never chmod() again
- the target path is UNIX domain socket (openat() returns ENXIO)
  => bind() of UNIX domain sockets fails if the file is on 9pfs

The solution is to use O_PATH: openat() now succeeds in both cases, and we
can ensure the path isn't a symlink with fstat(). The associated entry in
"/proc/self/fd" can hence be safely passed to the regular chmod() syscall.

The previous behavior is kept for older systems that don't have O_PATH.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Zhi Yong Wu <zhiyong.wu@ucloud.cn>
Acked-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

1 files changed, 15 insertions, 9 deletions

diff --git a/hw/9pfs/9p-util.h b/hw/9pfs/9p-util.h
index 91299a24b8..dc0d2e29aa 100644
--- a/hw/9pfs/9p-util.h
+++ b/hw/9pfs/9p-util.h
@@ -13,6 +13,12 @@
 #ifndef QEMU_9P_UTIL_H
 #define QEMU_9P_UTIL_H
 
+#ifdef O_PATH
+#define O_PATH_9P_UTIL O_PATH
+#else
+#define O_PATH_9P_UTIL 0
+#endif
+
 static inline void close_preserve_errno(int fd)
 {
     int serrno = errno;
@@ -22,13 +28,8 @@ static inline void close_preserve_errno(int fd)
 
 static inline int openat_dir(int dirfd, const char *name)
 {
-#ifdef O_PATH
-#define OPENAT_DIR_O_PATH O_PATH
-#else
-#define OPENAT_DIR_O_PATH 0
-#endif
     return openat(dirfd, name,
-                  O_DIRECTORY | O_RDONLY | O_NOFOLLOW | OPENAT_DIR_O_PATH);
+                  O_DIRECTORY | O_RDONLY | O_NOFOLLOW | O_PATH_9P_UTIL);
 }
 
 static inline int openat_file(int dirfd, const char *name, int flags,
@@ -43,9 +44,14 @@ static inline int openat_file(int dirfd, const char *name, int flags,
     }
 
     serrno = errno;
-    /* O_NONBLOCK was only needed to open the file. Let's drop it. */
-    ret = fcntl(fd, F_SETFL, flags);
-    assert(!ret);
+    /* 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()
+     * ignored it anyway.
+     */
+    if (!(flags & O_PATH_9P_UTIL)) {
+        ret = fcntl(fd, F_SETFL, flags);
+        assert(!ret);
+    }
     errno = serrno;
     return fd;
 }