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Date:   Tue, 29 Sep 2020 16:05:49 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Marco Elver <elver@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Potapenko <glider@google.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
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        Andrey Ryabinin <aryabinin@virtuozzo.com>,
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        Borislav Petkov <bp@alien8.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
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Subject: Re: [PATCH v3 01/10] mm: add Kernel Electric-Fence infrastructure
Message-ID: <20200929150549.GE53442@C02TD0UTHF1T.local>
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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@arm.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).

Other than the code to initialize the page tables for the careveout, I
think the carevout code can be geenric.

Thanks,
Mark.