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

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Message-ID: <CAFULd4ZCdDGxMZiPPA8q9AGi=UEdNWhqw6ndggX1uw=LP9+vxw@mail.gmail.com>
Date: Thu, 20 Feb 2025 21:09:42 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Brian Gerst <brgerst@...il.com>
Cc: Ard Biesheuvel <ardb@...nel.org>, 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 6:59 PM Brian Gerst <brgerst@...il.com> wrote:
>
> 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.

Not really. If we put stack_canary as the first member of struct
pcpu_hot, we can just alias __stack_chk_guard to struct pcpu_hot in
the linker script, and everything starts to magically work, SMP and
!SMP. Please see the proposed patch, effectively a three liner, at
[1].

[1] https://lore.kernel.org/lkml/20250220200439.4458-1-ubizjak@gmail.com/

Uros.