Commit 2ef3cec4 authored by Alexander Potapenko's avatar Alexander Potapenko Committed by Andrew Morton
Browse files

kmsan: do not wipe out origin when doing partial unpoisoning 

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.

Link: https://lkml.kernel.org/r/20240528104807.738758-1-glider@google.com


Fixes: f80be457 ("kmsan: add KMSAN runtime core")
Signed-off-by: default avatarAlexander Potapenko <glider@google.com>
Reported-by: default avatarBrian Johannesmeyer <bjohannesmeyer@gmail.com>
  Link: https://lore.kernel.org/lkml/20240524232804.1984355-1-bjohannesmeyer@gmail.com/T/


Reviewed-by: default avatarMarco Elver <elver@google.com>
Tested-by: default avatarBrian Johannesmeyer <bjohannesmeyer@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 0105eaab

mm/kmsan/core.c

+11 −4
Original line number Diff line number Diff line
@@ -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));
@@ -225,9 +224,17 @@ 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++)

	/*

	 * 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)

{