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

From: Marco Elver
Date: Tue May 28 2024 - 08:38:54 EST


On Tue, 28 May 2024 at 12:48, Alexander Potapenko <glider@xxxxxxxxxx> 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@xxxxxxxxx>
> Link: https://lore.kernel.org/lkml/20240524232804.1984355-1-bjohannesmeyer@xxxxxxxxx/T/
> Fixes: f80be4571b19 ("kmsan: add KMSAN runtime core")
> Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
> ---
> 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;
> + }

Reviewed-by: Marco Elver <elver@xxxxxxxxxx>