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

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Message-ID: <06a7c031-3ef9-7f8d-6292-18e2cca0698a@linux.intel.com>
Date:   Thu, 11 Jan 2018 10:35:57 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Willy Tarreau <w@....eu>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        Ingo Molnar <mingo@...nel.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 01/11/2018 07:44 AM, Willy Tarreau wrote:
>> 4. Cleared on setuid() and friends
> This one causes me a problem : some daemons already take care of dropping
> privileges after the initial fork() for the sake of security. Haproxy
> typically does this at boot :
> 
>    - parse config
>    - chroot to /var/empty
>    - setuid(dedicated_uid)
>    - fork()

This makes me a _bit_ nervous.  I think Andy touched on this, but I'll
say it another way: you want PTI turned off because you trust an app to
be good, but you also drop permissions because it is exposed to an
environment where you do *not* fully trust it.

I'm not sure how you reconcile that.

If your proxy gets compromised, and pti is turned off, you are entirely
exposed to meltdown from that process.  I don't know exactly what you
are doing, but isn't this proxy sitting there shuffling untrusted user
data around all day?