Re: [PATCH v3 1/4] kasan, arm64: Add KASAN light mode

Hi Mark,

On 1/15/21 3:08 PM, Mark Rutland wrote:

On Fri, Jan 15, 2021 at 12:00:40PM +0000, Vincenzo Frascino wrote:

quoted

Architectures supported by KASAN HW can provide a light mode of
execution. On an MTE enabled arm64 hw for example this can be identified
with the asynch mode of execution.
In this mode, if a tag check fault occurs, the TFSR_EL1 register is
updated asynchronously. The kernel checks the corresponding bits
periodically.

What's the expected usage of this relative to prod, given that this has
to be chosen at boot time? When/where is this expected to be used
relative to prod mode?

IIUC the light mode is meant for low spec devices. I let Andrey comment a bit
more on this topic.

quoted

diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index 18fce223b67b..3a7c5beb7096 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h

@@ -231,7 +231,7 @@ static inline const void *__tag_set(const void *addr, u8 tag)
 }
 
 #ifdef CONFIG_KASAN_HW_TAGS
-#define arch_enable_tagging()			mte_enable_kernel()
+#define arch_enable_tagging(mode)		mte_enable_kernel(mode)

Rather than passing a mode in, I think it'd be better to have:

* arch_enable_tagging_prod()
* arch_enable_tagging_light()

... that we can map in the arch code to separate:

* mte_enable_kernel_sync()
* mte_enable_kernel_async()

... as by construction that avoids calls with an unhandled mode, and we
wouldn't need the mode enum kasan_hw_tags_mode...

quoted

+static inline int hw_init_mode(enum kasan_arg_mode mode)
+{
+	switch (mode) {
+	case KASAN_ARG_MODE_LIGHT:
+		return KASAN_HW_TAGS_ASYNC;
+	default:
+		return KASAN_HW_TAGS_SYNC;
+	}
+}

... and we can just have a wrapper like this to call either of the two functions directly, i.e.

static inline void hw_enable_tagging_mode(enum kasan_arg_mode mode)
{
	if (mode == KASAN_ARG_MODE_LIGHT)
		arch_enable_tagging_mode_light();
	else
		arch_enable_tagging_mode_prod();
}

Fine by me, this would remove the need of adding a new enumeration as well and
reflect on the arch code. I would keep "arch_enable_tagging_mode_sync" and
"arch_enable_tagging_mode_async" though to give a clear indication in the KASAN
code of the mode we are setting. I will adapt my code accordingly for v4.

Thanks,
Mark.

-- 
Regards,
Vincenzo

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