linux-kernel - Re: [PATCH v3 01/10] mm: add Kernel Electric-Fence infrastructure

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Date:   Tue, 29 Sep 2020 16:51:29 +0200
From:   Marco Elver <elver@...gle.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoph Lameter <cl@...ux.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hillf Danton <hdanton@...a.com>,
        Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Jonathan Corbet <corbet@....net>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Kees Cook <keescook@...omium.org>,
        Pekka Enberg <penberg@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        SeongJae Park <sjpark@...zon.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH v3 01/10] mm: add Kernel Electric-Fence infrastructure

On Tue, 29 Sep 2020 at 16:24, Mark Rutland <mark.rutland@....com> wrote:
[...]
>
> From other sub-threads it sounds like these addresses are not part of
> the linear/direct map. Having kmalloc return addresses outside of the
> linear map is going to break anything that relies on virt<->phys
> conversions, and is liable to make DMA corrupt memory. There were
> problems of that sort with VMAP_STACK, and this is why kvmalloc() is
> separate from kmalloc().
>
> Have you tested with CONFIG_DEBUG_VIRTUAL? I'd expect that to scream.
>
> I strongly suspect this isn't going to be safe unless you always use an
> in-place carevout from the linear map (which could be the linear alias
> of a static carevout).

That's an excellent point, thank you! Indeed, on arm64, a version with
naive static-pool screams with CONFIG_DEBUG_VIRTUAL.

We'll try to put together an arm64 version using a carveout as you suggest.

> [...]
>
> > +static __always_inline void *kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
> > +{
> > +     return static_branch_unlikely(&kfence_allocation_key) ? __kfence_alloc(s, size, flags) :
> > +                                                                   NULL;
> > +}
>
> Minor (unrelated) nit, but this would be easier to read as:
>
> static __always_inline void *kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
> {
>         if (static_branch_unlikely(&kfence_allocation_key))
>                 return __kfence_alloc(s, size, flags);
>         return NULL;
> }

Will fix for v5.

Thanks,
-- Marco