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

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Message-ID: <CAMj1kXGG4J76o17d=e4jv-5jjzcSGyZdKRcNNB17SkCqD6+8qQ@mail.gmail.com>
Date: Thu, 20 Feb 2025 11:52:23 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Uros Bizjak <ubizjak@...il.com>
Cc: Brian Gerst <brgerst@...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, 20 Feb 2025 at 11:46, Uros Bizjak <ubizjak@...il.com> wrote:
>
> On Thu, Feb 20, 2025 at 11:05 AM Ard Biesheuvel <ardb@...nel.org> wrote:
> >
> > Hi Uros,
> >
> > On Thu, 20 Feb 2025 at 10:51, Uros Bizjak <ubizjak@...il.com> wrote:
> > >
> > > On Wed, Feb 19, 2025 at 2:18 PM Brian Gerst <brgerst@...il.com> wrote:
> > > >
> > > > On Wed, Feb 19, 2025 at 6:47 AM Ingo Molnar <mingo@...nel.org> wrote:
> > > > >
> > > > > * Uros Bizjak <ubizjak@...il.com> wrote:
> > > > >
> > > > > > I wonder if there would be any benefit if stack canary is put into
> > > > > > struct pcpu_hot?
> > > > >
> > > > > It should definitely be one of the hottest data structures on x86, so
> > > > > moving it there makes sense even if it cannot be measured explicitly.
> > > > >
> > > >
> > > > It would have to be done with linker tricks, since you can't make the
> > > > compiler use a struct member directly.
> > >
> >
> > Interesting take. I'd have tried to put the canary at offset 0x0, and
> > simply use pcpu_hot as the guard symbol.
> >
> >
> > > It boots and runs without problems.
> > >
> > > However, when building the kernel, I get "Absolute relocations
> > > present" warning with thousands of locations:
> > >
...
> >
> > The warning is about the type of __ref_stack_chk_guard, not about the
> > type of the relocation.
>
> Thanks, I got distracted by the text of the warning that mentions relocation.
>
> > $ nm vmlinux |grep \\s__ref_sta
> > ffffffff8350c620 A __ref_stack_chk_guard
> >
> > Without your patch:
> >
> > $ nm vmlinux |grep \\s__ref_sta
> > ffffffff834fba10 D __ref_stack_chk_guard
>
> Is this a problem in our specific case?

I don't think so - the whole notion of absolute ELF symbols is rather
flaky IME, so I don't think we should be pedantic here.

> We can list the symbol in arch/x86/tools/relocs.c to quiet the
> warning, but I would need some help with auditing the symbol itself.
>
> 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.