[PATCH AUTOSEL 5.4 257/330] mm/slub: fix incorrect interpretation of s->offset

Sasha Levin
2020-09-18T02:57:55Z

From: Waiman Long <longman@redhat.com>

[ Upstream commit cbfc35a48609ceac978791e3ab9dde0c01f8cb20 ]

In a couple of places in the slub memory allocator, the code uses
"s->offset" as a check to see if the free pointer is put right after the
object. That check is no longer true with commit 3202fa62fb43 ("slub:
relocate freelist pointer to middle of object").

As a result, echoing "1" into the validate sysfs file, e.g. of dentry,
may cause a bunch of "Freepointer corrupt" error reports like the
following to appear with the system in panic afterwards.

=============================================================================
BUG dentry(666:pmcd.service) (Tainted: G B): Freepointer corrupt
-----------------------------------------------------------------------------

To fix it, use the check "s->offset == s->inuse" in the new helper
function freeptr_outside_object() instead. Also add another helper
function get_info_end() to return the end of info block (inuse + free
pointer if not overlapping with object).

Fixes: 3202fa62fb43 ("slub: relocate freelist pointer to middle of object")
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Rafael Aquini <aquini@redhat.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Vitaly Nikolenko <vnik@duasynt.com>
Cc: Silvio Cesare <silvio.cesare@gmail.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Markus Elfring <Markus.Elfring@web.de>
Cc: Changbin Du <changbin.du@gmail.com>
Link: http://lkml.kernel.org/r/20200429135328.26976-1-longman@redhat.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
mm/slub.c | 45 ++++++++++++++++++++++++++++++---------------
1 file changed, 30 insertions(+), 15 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 822ba07245291..d69934eac9e94 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -533,15 +533,32 @@ static void print_section(char *level, char *text, u8 *addr,
metadata_access_disable();
}

+/*
+ * See comment in calculate_sizes().
+ */
+static inline bool freeptr_outside_object(struct kmem_cache *s)
+{
+ return s->offset >= s->inuse;
+}
+
+/*
+ * Return offset of the end of info block which is inuse + free pointer if
+ * not overlapping with object.
+ */
+static inline unsigned int get_info_end(struct kmem_cache *s)
+{
+ if (freeptr_outside_object(s))
+ return s->inuse + sizeof(void *);
+ else
+ return s->inuse;
+}
+
static struct track *get_track(struct kmem_cache *s, void *object,
enum track_item alloc)
{
struct track *p;

- if (s->offset)
- p = object + s->offset + sizeof(void *);
- else
- p = object + s->inuse;
+ p = object + get_info_end(s);

return p + alloc;
}
@@ -682,10 +699,7 @@ static void print_trailer(struct kmem_cache *s, struct page *page, u8 *p)
print_section(KERN_ERR, "Redzone ", p + s->object_size,
s->inuse - s->object_size);

- if (s->offset)
- off = s->offset + sizeof(void *);
- else
- off = s->inuse;
+ off = get_info_end(s);

if (s->flags & SLAB_STORE_USER)
off += 2 * sizeof(struct track);
@@ -776,7 +790,7 @@ static int check_bytes_and_report(struct kmem_cache *s, struct page *page,
* object address
* Bytes of the object to be managed.
* If the freepointer may overlay the object then the free
- * pointer is the first word of the object.
+ * pointer is at the middle of the object.
*
* Poisoning uses 0x6b (POISON_FREE) and the last byte is
* 0xa5 (POISON_END)
@@ -810,11 +824,7 @@ static int check_bytes_and_report(struct kmem_cache *s, struct page *page,

static int check_pad_bytes(struct kmem_cache *s, struct page *page, u8 *p)
{
- unsigned long off = s->inuse; /* The end of info */
-
- if (s->offset)
- /* Freepointer is placed after the object. */
- off += sizeof(void *);
+ unsigned long off = get_info_end(s); /* The end of info */

if (s->flags & SLAB_STORE_USER)
/* We also have user information there */
@@ -900,7 +910,7 @@ static int check_object(struct kmem_cache *s, struct page *page,
check_pad_bytes(s, page, p);
}

- if (!s->offset && val == SLUB_RED_ACTIVE)
+ if (!freeptr_outside_object(s) && val == SLUB_RED_ACTIVE)
/*
* Object and freepointer overlap. Cannot check
* freepointer while object is allocated.
@@ -3585,6 +3595,11 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
*
* This is the case if we do RCU, have a constructor or
* destructor or are poisoning the objects.
+ *
+ * The assumption that s->offset >= s->inuse means free
+ * pointer is outside of the object is used in the
+ * freeptr_outside_object() function. If that is no
+ * longer true, the function needs to be modified.
*/
s->offset = size;
size += sizeof(void *);
--
2.25.1