linux-kernel - RE: [RFC PATCH] x86/entry/64: randomize kernel stack offset upon syscall

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Date:   Wed, 20 Mar 2019 11:12:07 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Andy Lutomirski' <luto@...nel.org>,
        Elena Reshetova <elena.reshetova@...el.com>
CC:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>,
        "Perla, Enrico" <enrico.perla@...el.com>,
        "Ingo Molnar" <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: RE: [RFC PATCH] x86/entry/64: randomize kernel stack offset upon
 syscall

From: Andy Lutomirski
> Sent: 18 March 2019 20:16
...
> > As a result this patch introduces 8 bits of randomness
> > (bits 4 - 11 are randomized, bits 0-3 must be zero due to stack alignment)
> > after pt_regs location on the thread stack.
> > The amount of randomness can be adjusted based on how much of the
> > stack space we wish/can trade for security.
> 
> Why do you need four zero bits at the bottom?  x86_64 Linux only
> maintains 8 byte stack alignment.

ISTR that the gcc developers arbitrarily changed the alignment
a few years ago.
If the stack is only 8 byte aligned and you allocate a variable that
requires 16 byte alignment you need gcc to generate the extra stack
frame to align the stack.
I don't remember seeing the relevant gcc options on the linux
gcc command lines.

	David

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