Re: [PATCH] UML: add support for KASAN under x86_64

[Johannes Berg <johannes@sipsolutions.net>]

Hi Vincent,

> Old thread, but I had a look at this the other day and I think I got it
> working.  Since the entire shadow area is mapped at init, we don't need
> to do any mappings later.

Nice!! I've always wanted to get back to this too.

> It works both with and without KASAN_VMALLOC.  KASAN_STACK works too
> after I disabled sanitization of the stacktrace code.  All kasan kunit
> tests pass and the test_kasan.ko module works too.

:-)

> The CONFIG_UML checks need to
> be replaced with something more appropriate (new config? __weak
> functions?) and the free functions should probably be hooked up to
> madvise(MADV_DONTNEED) so we discard unused pages in the shadow
> mapping.

I guess a new config would be most appropriate - that way code can be
compiled out accordingly. But I don't know who maintains the KASAN code,
I guess have to discuss with them.

> Note that there's a KASAN stack-out-of-bounds splat on startup when just
> booting UML.  That looks like a real (17-year-old) bug, I've posted a
> fix for that:
> 
>  https://lore.kernel.org/lkml/20220523140403.2361040-1-vincent.whitchurch@axis.com/

Hah, right, I was wondering how that came up suddenly now... Almost
suprised it's just a single bug so far :)

> --- a/mm/kasan/shadow.c
> +++ b/mm/kasan/shadow.c
> @@ -295,8 +295,14 @@ int kasan_populate_vmalloc(unsigned long addr, unsigned long size)
>  		return 0;
>  
>  	shadow_start = (unsigned long)kasan_mem_to_shadow((void *)addr);
> -	shadow_start = ALIGN_DOWN(shadow_start, PAGE_SIZE);
>  	shadow_end = (unsigned long)kasan_mem_to_shadow((void *)addr + size);
> +
> +	if (IS_ENABLED(CONFIG_UML)) {
> +		__memset(kasan_mem_to_shadow((void *)addr), KASAN_VMALLOC_INVALID, shadow_end - shadow_start);
> +		return 0;
> +	}
> 

If that were

	if (IS_ENABLED(CONFIG_KASAN_NO_SHADOW_ALLOC)) {
		...
	}

(or so) as discussed above, it might be a little more readable, but
otherwise it doesn't really seem all _that_ intrusive.

I'll give it a spin later.

johannes