linux-kernel - Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when pti

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Date:   Wed, 10 Jan 2018 11:44:43 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Willy Tarreau <w@....eu>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when
 pti_disable is set

On Wed, Jan 10, 2018 at 11:39 AM, Willy Tarreau <w@....eu> wrote:
> Hi Andy,
>
> On Wed, Jan 10, 2018 at 11:21:15AM -0800, Andy Lutomirski wrote:
>> > If we agree on this, I'd like to propose to have two flags :
>> >
>> >   - TIF_DISABLE_PTI_NOW : disable PTI for the current task, reset by execve()
>> >   - TIF_DISABLE_PTI_NEXT : disable PTI after execve(), reset by execve()
>>
>> I really dislike state that isn't cleared on execve().  I'm assuming
>> that this is so you can run time pwn_me_without_pti whatever?
>
> Yes exactly. I've just sent a 3rd series with an example code for this.
> In fact it's not that the state is not cleared by execve(), it's that
> it's set for the next execve() which then resets it.
>
>> Surely LD_PRELOAD can do this, too?
>
> That was one of my other proposals. I really don't know if LD_PRELOAD
> fits anyone's usage for such things (static/setuid binaries, complication
> to pass variables maybe).
>
> Please take a look and tell me if you still dislike it or not.
>

Adding flags that persist across execve() of setuid stuff is IMO even
worse.  It just makes reasoning about the system's security model
really nasty.