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/[email protected]
Fixes: f80be45 ("kmsan: add KMSAN runtime core")
Signed-off-by: Alexander Potapenko <[email protected]>
Reported-by: Brian Johannesmeyer <[email protected]>
  Link: https://lore.kernel.org/lkml/[email protected]/T/
Reviewed-by: Marco Elver <[email protected]>
Tested-by: Brian Johannesmeyer <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: ramosian-glider

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)