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: f80be4571b19 ("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>

diff --git a/mm/kmsan/core.c b/mm/kmsan/core.c
index cf2d70e9c9a5ff985947859f3783fd8d5f0d9636..95f859e38c533d57f5ae637d69ad95bea4a1029e 100644 (file)
--- 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)