linux-kernel - [PATCH] mm/slub: Fix incorrect checkings of s->offset

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Date:   Sun, 26 Apr 2020 22:02:12 -0400
From:   Waiman Long <longman@...hat.com>
To:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Changbin Du <changbin.du@...il.com>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH] mm/slub: Fix incorrect checkings of s->offset

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 to appear with
the system in panic afterwards.

To fix it, use the check "s->offset == s->inuse" instead.

Fixes: 3202fa62fb43 ("slub: relocate freelist pointer to middle of object")
Signed-off-by: Waiman Long <longman@...hat.com>
---
 mm/slub.c | 18 +++++++-----------
 1 file changed, 7 insertions(+), 11 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 0e736d66bb42..99952d01e7e0 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -556,10 +556,8 @@ static struct track *get_track(struct kmem_cache *s, void *object,
 {
 	struct track *p;
 
-	if (s->offset)
-		p = object + s->offset + sizeof(void *);
-	else
-		p = object + s->inuse;
+	p = object + s->inuse +
+	    ((s->offset == s->inuse) ? sizeof(void *) : 0);
 
 	return p + alloc;
 }
@@ -693,10 +691,8 @@ 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 = s->inuse +
+	      ((s->offset == s->inuse) ? sizeof(void *) : 0);
 
 	if (s->flags & SLAB_STORE_USER)
 		off += 2 * sizeof(struct track);
@@ -790,7 +786,7 @@ 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)
@@ -826,7 +822,7 @@ 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)
+	if (s->offset == s->inuse)
 		/* Freepointer is placed after the object. */
 		off += sizeof(void *);
 
@@ -915,7 +911,7 @@ 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 ((s->offset != s->inuse) && val == SLUB_RED_ACTIVE)
 		/*
 		 * Object and freepointer overlap. Cannot check
 		 * freepointer while object is allocated.
-- 
2.18.1