[PATCH v2 15/16] mm: Make ksize() a reporting-only function

Kees Cook
2022-09-23T20:40:49Z

With all "silently resizing" callers of ksize() refactored, remove the
logic in ksize() that would allow it to be used to effectively change
the size of an allocation (bypassing __alloc_size hints, etc). Users
wanting this feature need to either use kmalloc_size_roundup() before an
allocation, or call krealloc() directly.

For kfree_sensitive(), move the unpoisoning logic inline. Replace the
open-coded ksize() in __do_krealloc with ksize() now that it doesn't
perform unpoisoning.

Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: linux-mm@kvack.org
Cc: kasan-dev@googlegroups.com
Signed-off-by: Kees Cook <keescook@chromium.org>
---
mm/slab_common.c | 38 ++++++++++++++------------------------
1 file changed, 14 insertions(+), 24 deletions(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index d7420cf649f8..60b77bcdc2e3 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1160,13 +1160,8 @@ __do_krealloc(const void *p, size_t new_size, gfp_t flags)
void *ret;
size_t ks;

- /* Don't use instrumented ksize to allow precise KASAN poisoning. */
- if (likely(!ZERO_OR_NULL_PTR(p))) {
- if (!kasan_check_byte(p))
- return NULL;
- ks = kfence_ksize(p) ?: __ksize(p);
- } else
- ks = 0;
+ /* How large is the allocation actually? */
+ ks = ksize(p);

/* If the object still fits, repoison it precisely. */
if (ks >= new_size) {
@@ -1232,8 +1227,10 @@ void kfree_sensitive(const void *p)
void *mem = (void *)p;

ks = ksize(mem);
- if (ks)
+ if (ks) {
+ kasan_unpoison_range(mem, ks);
memzero_explicit(mem, ks);
+ }
kfree(mem);
}
EXPORT_SYMBOL(kfree_sensitive);
@@ -1242,10 +1239,11 @@ EXPORT_SYMBOL(kfree_sensitive);
* ksize - get the actual amount of memory allocated for a given object
* @objp: Pointer to the object
*
- * kmalloc may internally round up allocations and return more memory
+ * kmalloc() may internally round up allocations and return more memory
* than requested. ksize() can be used to determine the actual amount of
- * memory allocated. The caller may use this additional memory, even though
- * a smaller amount of memory was initially specified with the kmalloc call.
+ * allocated memory. The caller may NOT use this additional memory, unless
+ * it calls krealloc(). To avoid an alloc/realloc cycle, callers can use
+ * kmalloc_size_roundup() to find the size of the associated kmalloc bucket.
* The caller must guarantee that objp points to a valid object previously
* allocated with either kmalloc() or kmem_cache_alloc(). The object
* must not be freed during the duration of the call.
@@ -1254,13 +1252,11 @@ EXPORT_SYMBOL(kfree_sensitive);
*/
size_t ksize(const void *objp)
{
- size_t size;
-
/*
- * We need to first check that the pointer to the object is valid, and
- * only then unpoison the memory. The report printed from ksize() is
- * more useful, then when it's printed later when the behaviour could
- * be undefined due to a potential use-after-free or double-free.
+ * We need to first check that the pointer to the object is valid.
+ * The KASAN report printed from ksize() is more useful, then when
+ * it's printed later when the behaviour could be undefined due to
+ * a potential use-after-free or double-free.
*
* We use kasan_check_byte(), which is supported for the hardware
* tag-based KASAN mode, unlike kasan_check_read/write().
@@ -1274,13 +1270,7 @@ size_t ksize(const void *objp)
if (unlikely(ZERO_OR_NULL_PTR(objp)) || !kasan_check_byte(objp))
return 0;

- size = kfence_ksize(objp) ?: __ksize(objp);
- /*
- * We assume that ksize callers could use whole allocated area,
- * so we need to unpoison this area.
- */
- kasan_unpoison_range(objp, size);
- return size;
+ return kfence_ksize(objp) ?: __ksize(objp);
}
EXPORT_SYMBOL(ksize);

--
2.34.1