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From:   Alexander Potapenko <glider@google.com>
Date:   Thu, 1 Oct 2020 13:24:49 +0200
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Subject: Re: [PATCH v3 03/10] arm64, kfence: enable KFENCE for ARM64
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     Will Deacon <will@kernel.org>, Marco Elver <elver@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
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Mark,

> If you need virt_to_page() to work, the address has to be part of the
> linear/direct map.
>
> If you need to find the struct page for something that's part of the
> kernel image you can use virt_to_page(lm_alias(x)).
>
> > Looks like filling page table entries (similarly to what's being done
> > in arch/arm64/mm/kasan_init.c) is not enough.
> > I thought maybe vmemmap_populate() would do the job, but it didn't
> > (virt_to_pfn() still returns invalid PFNs).
>
> As above, I think lm_alias() will solve the problem here. Please see
> that and CONFIG_DEBUG_VIRTUAL.

The approach you suggest works to some extent, but there are some caveats.

To reiterate, we are trying to allocate the pool (2Mb by default, but
users may want a bigger one, up to, say, 64 Mb) in a way that:
(1) The underlying page tables support 4K granularity.
(2) is_kfence_address() (checks that __kfence_pool <= addr <=
__kfence_pool + KFENCE_POOL_SIZE) does not reference memory
(3) For addresses belonging to that pool virt_addr_valid() is true
(SLAB/SLUB rely on that)

On x86 we achieve (2) by making our pool a .bss array, so that its
address is known statically. Aligning that array on 4K and calling
set_memory_4k() ensures that (1) is also fulfilled. (3) seems to just
happen automagically without any address translations.

Now, what we are seeing on arm64 is different.
My understanding (please correct me if I'm wrong) is that on arm64
only the memory range at 0xffff000000000000 has valid struct pages,
and the size of that range depends on the amount of memory on the
system.
This probably means we cannot just pick a fixed address for our pool
in that range, unless it is very close to 0xffff000000000000.

If we allocate the pool statically in the way x86 does (assuming we
somehow resolve (1)), we can apply lm_alias() to addresses returned by
the KFENCE allocator, making kmalloc() always return addresses from
the linear map and satisfying (3).
But in that case is_kfence_address() will also need to be updated to
compare the addresses to lm_alias(__kfence_pool), and this becomes
more heavyweight than just reading the address from memory.

So looks like it's still more preferable to allocate the pool
dynamically on ARM64, unless there's a clever trick to allocate a
fixed address in the linear map (DTS maybe?)

Thanks,
Alex