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

[Uros Bizjak]

On Thu, Feb 20, 2025 at 11:05 AM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> Hi Uros,
>
> On Thu, 20 Feb 2025 at 10:51, Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
> >
> > On Wed, Feb 19, 2025 at 2:18 PM Brian Gerst <brgerst@xxxxxxxxx> wrote:
> > >
> > > On Wed, Feb 19, 2025 at 6:47 AM Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> > > >
> > > >
> > > > * Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
> > > >
> > > > > > Thank you for doing this series - it all looks pretty good from my
> > > > > > side and I've applied it experimentally to tip:x86/asm. I fixed up
> > > > > > the trivial details other reviewers and me noticed.
> > > > > >
> > > > > > Note that the merge is tentative, it might still need a rebase if
> > > > > > some fundamental problem comes up - but let's see how testing goes
> > > > > > in -next.
> > > > >
> > > > > 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.
> >
> > Something like the attached patch?
> >
>
> 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:
> >
> >   RELOCS  arch/x86/boot/compressed/vmlinux.relocs
> > WARNING: Absolute relocations present
> > Offset     Info     Type     Sym.Value Sym.Name
> > ffffffff81200826 0003259e00000002 R_X86_64_PC32 ffffffff8350f620
> > __ref_stack_chk_guard
> > ffffffff81201493 0003259e00000002 R_X86_64_PC32 ffffffff8350f620
> > __ref_stack_chk_guard
> > ffffffff81201714 0003259e00000002 R_X86_64_PC32 ffffffff8350f620
> > __ref_stack_chk_guard
> > ffffffff81201d66 0003259e00000002 R_X86_64_PC32 ffffffff8350f620
> > __ref_stack_chk_guard
> > ...
> > ffffffff834e2a13 0003259e00000002 R_X86_64_PC32 ffffffff8350f620
> > __ref_stack_chk_guard
> > ffffffff834e2a6a 0003259e00000002 R_X86_64_PC32 ffffffff8350f620
> > __ref_stack_chk_guard
> >
> >   RSTRIP  vmlinux
> >
> > which I don't understand. Looking at the first one:
> >
> > ffffffff8120081d <force_ibs_eilvt_setup.cold>:
> > ffffffff8120081d:    48 8b 44 24 08           mov    0x8(%rsp),%rax
> > ffffffff81200822:    65 48 2b 05 f6 ed 30     sub
> > %gs:0x230edf6(%rip),%rax        # ffffffff8350f620
> > <__ref_stack_chk_guard>
> > ffffffff81200829:    02
> >
> > I don't think this is absolute relocation, see (%rip).
> >
>
> 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? While the symbol is absolute,
the relocation is still relative, so IMO it should be OK even with
your ongoing rip-relative efforts in mind.

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.

Uros.