linux-kernel - Re: [PATCH v6 00/15] x86-64: Stack protector and percpu improvements

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Message-ID: <CAMzpN2gvzVq9ASD7UEooMyH9Er0TpJ8dgeggWnBvZEMdy_4UpQ@mail.gmail.com>
Date: Thu, 20 Feb 2025 12:59:15 -0500
From: Brian Gerst <brgerst@...il.com>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Uros Bizjak <ubizjak@...il.com>, Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org, 
	x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v6 00/15] x86-64: Stack protector and percpu improvements

On Thu, Feb 20, 2025 at 12:47 PM Brian Gerst <brgerst@...il.com> wrote:
>
> On Thu, Feb 20, 2025 at 12:36 PM Ard Biesheuvel <ardb@...nel.org> wrote:
> >
> > On Thu, 20 Feb 2025 at 18:24, Brian Gerst <brgerst@...il.com> wrote:
> > >
> > > On Thu, Feb 20, 2025 at 5:52 AM Ard Biesheuvel <ardb@...nel.org> wrote:
> > > >
> > > > On Thu, 20 Feb 2025 at 11:46, Uros Bizjak <ubizjak@...il.com> wrote:
> > > > >
> > > > > OTOH, we could simply do it your way and put stack canary at the
> > > > > beginning of pcpu_hot structure, with
> > > > >
> > > > > static_assert(offsetof(struct pcpu_hot, stack_canary) == 0));
> > > > >
> > > > > for good measure.
> > > >
> > > > I think this would be the most straight-forward if there are no other
> > > > locality concerns this might interfere with.
> > >
> > > I'd prefer it at the end of pcpu_hot, that way the disassembler
> > > doesn't latch on to the __stack_chk_guard symbol when referencing the
> > > other fields of pcpu_hot.
> > >
> >
> > __stack_chk_guard would no longer exist, only __ref_stack_chk_guard,
> > which would be equal to pcpu_hot.  We could just call that
> > __ref_pcpu_hot instead if it might cause confusion otherwise. (We
> > can't use pcpu_hot directly in -mstack-check-guard-symbol= for the
> > same reasons I had to add the indirection via __ref_stack_chk_guard)
>
> That works for me.

Maybe not.  One quirk of how GCC implements this is that
-mstack-protector-guard=global (used by !SMP builds) ignores the
-mstack-protector-guard-symbol option and always uses
__stack_chk_guard.  That makes things more challenging.


Brian Gerst