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: f80be45 ("kmsan: add KMSAN runtime core") Signed-off-by: Alexander Potapenko <glider@google.com> Reported-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com> Link: https://lore.kernel.org/lkml/20240524232804.1984355-1-bjohannesmeyer@gmail.com/T/ Reviewed-by: Marco Elver <elver@google.com> Tested-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com> Cc: Dmitry Vyukov <dvyukov@google.com> Cc: Kees Cook <keescook@chromium.org> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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)