linux-kernel - Re: [PATCH v2 1/2] scs: switch to vmapped shadow stacks

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Date:   Mon, 30 Nov 2020 12:03:09 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Will Deacon <will@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Kees Cook <keescook@...omium.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] scs: switch to vmapped shadow stacks

On Mon, Nov 30, 2020 at 3:44 AM Will Deacon <will@...nel.org> wrote:
>
> On Tue, Nov 24, 2020 at 11:59:39AM -0800, Sami Tolvanen wrote:
> > +void *scs_alloc(int node)
> >  {
> > -     void *s = kmem_cache_alloc_node(scs_cache, GFP_SCS, node);
> > +     int i;
> > +     void *s;
> > +
> > +     for (i = 0; i < NR_CACHED_SCS; i++) {
> > +             s = this_cpu_xchg(scs_cache[i], NULL);
> > +             if (s) {
> > +                     kasan_unpoison_vmalloc(s, SCS_SIZE);
> > +                     memset(s, 0, SCS_SIZE);
> > +                     goto out;
> > +             }
> > +     }
> > +
> > +     s = __vmalloc_node_range(SCS_SIZE, 1, VMALLOC_START, VMALLOC_END,
> > +                              GFP_SCS, PAGE_KERNEL, 0, node,
> > +                              __builtin_return_address(0));
> >
> >       if (!s)
> >               return NULL;
>
> Sorry I didn't spot this before, but if you put the xchg/vmalloc code
> into a new __scs_alloc() function then you can drop the label and this
> becomes:
>
>         s = __scs_alloc(...);
>         if (!s)
>                 return NULL;
>
>         *__scs_maghic(s) = SCS_ENG_MAGIC;
>         ...

Good point, I'll change this in v3.

Sami