[PATCH 1/3] mm/slab: Validate cache membership under freelist hardening

[Kees Cook]

When building under CONFIG_SLAB_FREELIST_HARDENING, it makes
sense to perform sanity-checking on the assumed slab cache during
kmem_cache_free() to make sure the kernel doesn't mix freelists across
slab caches and corrupt memory (as seen in the exploitation of flaws like
CVE-2018-9568[1]). Note that the prior code might WARN() but still corrupt
memory (i.e. return the assumed cache instead of the owned cache).

There is no noticeable performance impact (changes are within noise).
Measuring parallel kernel builds, I saw the following with
CONFIG_SLAB_FREELIST_HARDENED, before and after this patch:

before:

	Run times: 288.85 286.53 287.09 287.07 287.21
	Min: 286.53 Max: 288.85 Mean: 287.35 Std Dev: 0.79

after:

	Run times: 289.58 287.40 286.97 287.20 287.01
	Min: 286.97 Max: 289.58 Mean: 287.63 Std Dev: 0.99

Delta: 0.1% which is well below the standard deviation

[1] https://github.com/ThomasKing2014/slides/raw/master/Building%20universal%20Android%20rooting%20with%20a%20type%20confusion%20vulnerability.pdf

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 mm/slab.h | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/mm/slab.h b/mm/slab.h
index 43ac818b8592..4dafae2c8620 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -310,7 +310,7 @@ static inline bool is_root_cache(struct kmem_cache *s)
 static inline bool slab_equal_or_root(struct kmem_cache *s,
 				      struct kmem_cache *p)
 {
-	return true;
+	return s == p;
 }
 
 static inline const char *cache_name(struct kmem_cache *s)
@@ -363,18 +363,16 @@ static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
 	 * will also be a constant.
 	 */
 	if (!memcg_kmem_enabled() &&
+	    !IS_ENABLED(CONFIG_SLAB_FREELIST_HARDENED) &&
 	    !unlikely(s->flags & SLAB_CONSISTENCY_CHECKS))
 		return s;
 
 	page = virt_to_head_page(x);
 	cachep = page->slab_cache;
-	if (slab_equal_or_root(cachep, s))
-		return cachep;
-
-	pr_err("%s: Wrong slab cache. %s but object is from %s\n",
-	       __func__, s->name, cachep->name);
-	WARN_ON_ONCE(1);
-	return s;
+	WARN_ONCE(!slab_equal_or_root(cachep, s),
+		  "%s: Wrong slab cache. %s but object is from %s\n",
+		  __func__, s->name, cachep->name);
+	return cachep;
 }
 
 static inline size_t slab_ksize(const struct kmem_cache *s)
-- 
2.17.1