Re: [PATCH 0/5 v3] fs: Fixes for removing xid bits and security labels

[Casey Schaufler <casey@schaufler-ca.com>]

On 5/19/2015 2:46 AM, Jan Kara wrote:
>   Hello,
>
>   This is a third version of my patches to fix handling of s[ug]id bits and
> capabilities xattrs in VFS. There are a few issues I found:
>
> 1) MS_NOSEC handling is broken - we set it after each file_remove_suid() call.
>    However we needn't have removed suid bit simply because we have
>    CAP_SYS_FSID and further writes to the file from processes without this
>    capability still need to clear the suid bit.
> 2) file_remove_suid() is a misnomer since it also handles removing of
>    security labels. It is even more confusing because should_remove_suid()
>    doesn't return whether file_remove_suid() is needed or not.
> 3) On truncate we do clear suid bits but not security labels. According to
>    documentation in include/linux/security.h that's a bug but please correct
>    me if I'm wrong.

The only security module that provides a hook for inode_killpriv is the
capability module. That clears the file based capabilities. Neither of the
modules that use inode based labels (SELinux and Smack) provide an inode_killpriv
hook. The text in lsm_hooks.h (moved from security.h) says "similar security
labels". The file based capabilities *are* similar to the setuid bit. The
mandatory access control labels used by SELinux and Smack are *dissimilar*.
The text in lsm_hooks should say "attributes", not "labels".

So the code you have will work the way everyone wants it to. We just need
to stop saying "labels" when we mean something else.

> 4) ocfs2 doesn't clear capability xattrs - hard to fix, I left it alone for
>    now.
> 5) XFS didn't provide proper exclusion for clearing mode bits.
>
>   This series aims at fixing above issues. Al, can you please merge the
> patches? Thanks!
>
>   Changes since v2:
> * Rebased on top of current Linus' tree
> * Improved patch 1 to use inode_has_no_xattr() as Linus suggested
>
>   Changes since v1:
> * Removed bogus patch changing inode_set_flags()
> * Updated changelog of patch 4/5 to better explain why ->inode_killpriv
>   should be called
> * Included a fix for MS_NOSEC handling in this series.
>
> 								Honza
> --
> To unsubscribe from this list: send the line "unsubscribe linux-security-module" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>