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

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

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

local_lsetxattr() 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, 3 insertions, 0 deletions

diff --git a/hw/9pfs/9p-xattr.h b/hw/9pfs/9p-xattr.h
index 69a8b6b62e..7558970d85 100644
--- a/hw/9pfs/9p-xattr.h
+++ b/hw/9pfs/9p-xattr.h
@@ -31,6 +31,9 @@ typedef struct xattr_operations
 
 ssize_t local_getxattr_nofollow(FsContext *ctx, const char *path,
                                 const char *name, void *value, size_t size);
+ssize_t local_setxattr_nofollow(FsContext *ctx, const char *path,
+                                const char *name, void *value, size_t size,
+                                int flags);
 
 extern XattrOperations mapped_user_xattr;
 extern XattrOperations passthrough_user_xattr;