linux-kernel - Re: [PATCH 0/2] setgid hardening

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Message-ID: <20170125235924.GC23701@1wt.eu>
Date:   Thu, 26 Jan 2017 00:59:24 +0100
From:   Willy Tarreau <w@....eu>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     security@...nel.org, Konstantin Khlebnikov <koct9i@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        yalin wang <yalin.wang2010@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 0/2] setgid hardening

On Wed, Jan 25, 2017 at 01:06:50PM -0800, Andy Lutomirski wrote:
> The kernel has some dangerous behavior involving the creation and
> modification of setgid executables.  These issues aren't kernel
> security bugs per se, but they have been used to turn various
> filesystem permission oddities into reliably privilege escalation
> exploits.
> 
> See http://www.halfdog.net/Security/2015/SetgidDirectoryPrivilegeEscalation/
> for a nice writeup.
> 
> Let's fix them for real.

BTW I like this. I vaguely remember having played with this when I
was a student 2 decades ago on a system where /var/spool/mail was
3777 (yes, setgid+sticky) and the mail files were 660. You could
deposit a shell there, then execute it with mail's permissions and
access any mailbox. That was quite odd as a design choice. The
impacts are often limited unless you find other ways to escalate
but generally it's not really clean.

Willy