Re: [PATCH 1/2] kmsan: do not wipe out origin when doing partial unpoisoning

[Brian Johannesmeyer <bjohannesmeyer@gmail.com>]

On Tue, May 28, 2024 at 12:48:06PM +0200, Alexander Potapenko wrote:
> As noticed by Brian, KMSAN should not be zeroing the origin when
> unpoisoning parts of a four-byte uninitialized value, e.g.:
> 
>     char a[4];
>     kmsan_unpoison_memory(a, 1);
> 
> This led to false negatives, as certain poisoned values could receive zero
> origins, preventing those values from being reported.
> 
> To fix the problem, check that kmsan_internal_set_shadow_origin() writes
> zero origins only to slots which have zero shadow.
> 
> Reported-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com>
> Link: https://lore.kernel.org/lkml/20240524232804.1984355-1-bjohannesmeyer@gmail.com/T/
> Fixes: f80be4571b19 ("kmsan: add KMSAN runtime core")
> Signed-off-by: Alexander Potapenko <glider@google.com>
> ---
>  mm/kmsan/core.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/kmsan/core.c b/mm/kmsan/core.c
> index cf2d70e9c9a5f..95f859e38c533 100644
> --- a/mm/kmsan/core.c
> +++ b/mm/kmsan/core.c
> @@ -196,8 +196,7 @@ void kmsan_internal_set_shadow_origin(void *addr, size_t size, int b,
>  				      u32 origin, bool checked)
>  {
>  	u64 address = (u64)addr;
> -	void *shadow_start;
> -	u32 *origin_start;
> +	u32 *shadow_start, *origin_start;
>  	size_t pad = 0;
>  
>  	KMSAN_WARN_ON(!kmsan_metadata_is_contiguous(addr, size));
> @@ -225,8 +224,16 @@ void kmsan_internal_set_shadow_origin(void *addr, size_t size, int b,
>  	origin_start =
>  		(u32 *)kmsan_get_metadata((void *)address, KMSAN_META_ORIGIN);
>  
> -	for (int i = 0; i < size / KMSAN_ORIGIN_SIZE; i++)
> -		origin_start[i] = origin;
> +	/*
> +	 * If the new origin is non-zero, assume that the shadow byte is also non-zero,
> +	 * and unconditionally overwrite the old origin slot.
> +	 * If the new origin is zero, overwrite the old origin slot iff the
> +	 * corresponding shadow slot is zero.
> +	 */
> +	for (int i = 0; i < size / KMSAN_ORIGIN_SIZE; i++) {
> +		if (origin || !shadow_start[i])
> +			origin_start[i] = origin;
> +	}
>  }
>  
>  struct page *kmsan_vmalloc_to_page_or_null(void *vaddr)
> -- 
> 2.45.1.288.g0e0cd299f1-goog
> 

Tested-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com>