[PATCH 1/2] mm: krealloc: consider spare memory for __GFP_ZERO

Danilo Krummrich
2024-07-30T19:42:20Z

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.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
mm/slab_common.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 40b582a014b8..cff602cedf8e 100644
--- 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;
}

base-commit: 7c3dd6d99f2df6a9d7944ee8505b195ba51c9b68
--
2.45.2