mm: krealloc: consider spare memory for __GFP_ZERO

As long as krealloc() is called with __GFP_ZERO consistently, starting
with the initial memory allocation, __GFP_ZERO should be fully honored.

However, if for an existing allocation krealloc() is called with a
decreased size, it is not ensured that the spare portion the allocation is
zeroed.  Thus, if krealloc() is subsequently called with a larger size
again, __GFP_ZERO can't be fully honored, since we don't know the previous
size, but only the bucket size.

Example:

	buf = kzalloc(64, GFP_KERNEL);
	memset(buf, 0xff, 64);

	buf = krealloc(buf, 48, GFP_KERNEL | __GFP_ZERO);

	/* After this call the last 16 bytes are still 0xff. */
	buf = krealloc(buf, 64, GFP_KERNEL | __GFP_ZERO);

Fix this, by explicitly setting spare memory to zero, when shrinking an
allocation with __GFP_ZERO flag set or init_on_alloc enabled.

Link: https://lkml.kernel.org/r/20240812223707.32049-1-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 40b582a014b8f2f9a27027c1345972c699205e2e..cff602cedf8e636c57026784beceed2cbaf53efd 100644 (file)
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1273,6 +1273,13 @@ __do_krealloc(const void *p, size_t new_size, gfp_t flags)
 
        /* If the object still fits, repoison it precisely. */
        if (ks >= new_size) {
+               /* Zero out spare memory. */
+               if (want_init_on_alloc(flags)) {
+                       kasan_disable_current();
+                       memset((void *)p + new_size, 0, ks - new_size);
+                       kasan_enable_current();
+               }
+
                p = kasan_krealloc((void *)p, new_size, flags);
                return (void *)p;
        }