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

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Date:   Mon, 5 Oct 2020 18:00:57 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Marco Elver <elver@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "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, Sep 29, 2020 at 5:06 PM Mark Rutland <mark.rutland@....com> wrote:
>
> On Tue, Sep 29, 2020 at 04:51:29PM +0200, Marco Elver wrote:
> > 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.
>
> Great, thanks!
>
> Just to be clear, the concerns for DMA and virt<->phys conversions also
> apply to x86 (the x86 virt<->phys conversion behaviour is more forgiving
> in the common case, but still has cases that can go wrong).

To clarify, shouldn't kmalloc/kmem_cache allocations used with DMA be
allocated with explicit GFP_DMA?
If so, how practical would it be to just skip such allocations in
KFENCE allocator?