9pfs: local: lgetxattr: don't follow symlinks

The local_lgetxattr() callback is vulnerable to symlink attacks because
it calls lgetxattr() which follows symbolic links in all path elements but
the rightmost one.

This patch introduces a helper to emulate the non-existing fgetxattrat()
function: it is implemented with /proc/self/fd which provides a trusted
path that can be safely passed to lgetxattr().

local_lgetxattr() is converted to use this helper and opendir_nofollow().

This partly fixes CVE-2016-9602.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/hw/9pfs/9p-util.c b/hw/9pfs/9p-util.c
index 54134b0bb4..fdb4d57376 100644
--- a/hw/9pfs/9p-util.c
+++ b/hw/9pfs/9p-util.c
@@ -11,6 +11,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/xattr.h"
 #include "9p-util.h"
 
 int relative_openat_nofollow(int dirfd, const char *path, int flags,
@@ -55,3 +56,14 @@ int relative_openat_nofollow(int dirfd, const char *path, int flags,
 
     return fd;
 }
+
+ssize_t fgetxattrat_nofollow(int dirfd, const char *filename, const char *name,
+                             void *value, size_t size)
+{
+    char *proc_path = g_strdup_printf("/proc/self/fd/%d/%s", dirfd, filename);
+    int ret;
+
+    ret = lgetxattr(proc_path, name, value, size);
+    g_free(proc_path);
+    return ret;
+}