Re: [PATCH 2/3] arm64: kasan: mte: use a constant kernel GCR_EL1 value

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Jul 14, 2021 at 03:38:42PM +0100, Mark Rutland wrote:
> When KASAN_HW_TAGS is selected, KASAN is enabled at boot time, and the
> hardware supports MTE, we'll initialize `kernel_gcr_excl` with a value
> dependent on KASAN_TAG_MAX. While the resulting value is a constant
> which depends on KASAN_TAG_MAX, we have to perform some runtime work to
> generate the value, and have to read the value from memory during the
> exception entry path. It would be better if we could generate this as a
> constant at compile-time, and use it as such directly.
> 
> Early in boot within __cpu_setup(), we initialize GCR_EL1 to a safe
> value, and later override this with the value required by KASAN. If
> CONFIG_KASAN_HW_TAGS is not selected, or if KASAN is disabeld at boot
> time, the kernel will not use IRG instructions, and so the initial value
> of GCR_EL1 is does not matter to the kernel. Thus, we can instead have
> __cpu_setup() initialize GCR_EL1 to a value consistent with
> KASAN_TAG_MAX, and avoid the need to re-initialize it during hotplug and
> resume form suspend.
> 
> This patch makes arem64 use a compile-time constant KERNEL_GCR_EL1
> value, which is compatible with KASAN_HW_TAGS when this is selected.
> This removes the need to re-initialize GCR_EL1 dynamically, and acts as
> an optimization to the entry assembly, which no longer needs to load
> this value from memory. The redundant initialization hooks are removed.
> 
> In order to do this, KASAN_TAG_MAX needs to be visible outside of the
> core KASAN code. To do this, I've moved the KASAN_TAG_* values into
> <linux/kasan-tags.h>.
> 
> There should be no functional change as a result of this patch.
> 
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Alexander Potapenko <glider@google.com>
> Cc: Andrey Konovalov <andreyknvl@gmail.com>
> Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Peter Collingbourne <pcc@google.com>
> Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
> Cc: Will Deacon <will@kernel.org>

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

> diff --git a/include/linux/kasan-tags.h b/include/linux/kasan-tags.h
> new file mode 100644
> index 000000000000..4f85f562512c
> --- /dev/null
> +++ b/include/linux/kasan-tags.h
> @@ -0,0 +1,15 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_KASAN_TAGS_H
> +#define _LINUX_KASAN_TAGS_H
> +
> +#define KASAN_TAG_KERNEL	0xFF /* native kernel pointers tag */
> +#define KASAN_TAG_INVALID	0xFE /* inaccessible memory tag */
> +#define KASAN_TAG_MAX		0xFD /* maximum value for random tags */
> +
> +#ifdef CONFIG_KASAN_HW_TAGS
> +#define KASAN_TAG_MIN		0xF0 /* minimum value for random tags */
> +#else
> +#define KASAN_TAG_MIN		0x00 /* minimum value for random tags */
> +#endif
> +
> +#endif /* LINUX_KASAN_TAGS_H */

If the kasan folk are happy with this change, I can take the patches
through the arm64 tree.

-- 
Catalin

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