linux-kernel - Re: [REVIEW][PATCH 2/3] exec: Don't allow ptracing an exec of an unreadable file

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Date:   Thu, 17 Nov 2016 13:07:33 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Willy Tarreau <w@....eu>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Michal Hocko <mhocko@...nel.org>, Jann Horn <jann@...jh.net>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [REVIEW][PATCH 2/3] exec: Don't allow ptracing an exec of an
 unreadable file

On Thu, Nov 17, 2016 at 12:47 PM, Willy Tarreau <w@....eu> wrote:
> On Thu, Nov 17, 2016 at 11:08:22AM -0600, Eric W. Biederman wrote:
>>
>> It is the reasonable expectation that if an executable file is not
>> readable there will be no way for a user without special privileges to
>> read the file.  This is enforced in ptrace_attach but if we are
>> already attached there is no enforcement if a readonly executable
>> is exec'd.
>
> I'm really scared by this Eric. At least you want to make it a hardening
> option that can be disabled at run time, otherwise it can easily break a
> lot of userspace :
>
> admin@...ha:~$ ll /bin/bash /bin/coreutils /bin/ls /usr/bin/telnet
> -r-xr-x--x 1 root adm 549272 Oct 28 16:25 /bin/bash
> -rwx--x--x 1 root adm 765624 Oct 28 16:27 /bin/coreutils
> lrwxrwxrwx 1 root root 9 Oct 28 16:27 /bin/ls -> coreutils
> -r-xr-x--x 1 root adm  70344 Oct 28 16:34 /usr/bin/telnet
>
> And I've not invented it, I've being taught to do this more than 20
> years ago and been doing this since on any slightly hardened server
> just because in pratice it's efficient at stopping quite a bunch of
> rootkits which require to copy and modify your executables. Sure
> they could get the contents using ptrace, but using cp is much more
> common than ptrace in scripts and that works. This has prooven quite
> efficient in field at stopping some rootkits several times over the
> last two decades and I know I'm not the only one to do it. In fact
> I *never* install an executable with read permissions for users if
> there's no need for random users to copy it. Does it mean that
> nobody should be able to see why their favorite utility doesn't
> work anymore ? Not in my opinion, at least not by default.
>
> So here I fear that we'll break strace at many places where strace
> precisely matters to debug things.
>
> However I'd love to have this feature controlled by a sysctl (to
> enforce it by default where possible).

I'm not opposed to a sysctl for this. Regardless, I think we need to
embrace this idea now, though, since we'll soon end up with
architectures that enforce executable-only memory, in which case
ptrace will again fail. Almost better to get started here and then not
have more surprises later.

-Kees

-- 
Kees Cook
Nexus Security