ovl: update S_ISGID when setting posix ACLs

This change fixes xfstest generic/375, which failed to clear the
setgid bit in the following test case on overlayfs:

  touch $testfile
  chown 100:100 $testfile
  chmod 2755 $testfile
  _runas -u 100 -g 101 -- setfacl -m u::rwx,g::rwx,o::rwx $testfile

Reported-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Tested-by: Amir Goldstein <amir73il@gmail.com>
Fixes: d837a49bd57f ("ovl: fix POSIX ACL setting")
Cc: <stable@vger.kernel.org> # v4.8

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index bcf3965be81942415c404b1253bfd59f809c0100..edd46a0e951d3e6d5ecbae0de8091c6f903d5297 100644 (file)
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -1037,6 +1037,21 @@ ovl_posix_acl_xattr_set(const struct xattr_handler *handler,
 
        posix_acl_release(acl);
 
+       /*
+        * Check if sgid bit needs to be cleared (actual setacl operation will
+        * be done with mounter's capabilities and so that won't do it for us).
+        */
+       if (unlikely(inode->i_mode & S_ISGID) &&
+           handler->flags == ACL_TYPE_ACCESS &&
+           !in_group_p(inode->i_gid) &&
+           !capable_wrt_inode_uidgid(inode, CAP_FSETID)) {
+               struct iattr iattr = { .ia_valid = ATTR_KILL_SGID };
+
+               err = ovl_setattr(dentry, &iattr);
+               if (err)
+                       return err;
+       }
+
        err = ovl_xattr_set(dentry, handler->name, value, size, flags);
        if (!err)
                ovl_copyattr(ovl_inode_real(inode, NULL), inode);