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From:   Marco Elver <elver@google.com>
Date:   Tue, 29 Sep 2020 16:51:29 +0200
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Subject: Re: [PATCH v3 01/10] mm: add Kernel Electric-Fence infrastructure
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Potapenko <glider@google.com>,
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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.

> [...]
>
> > +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