linux-hardening - [PATCH 4/4] mm/util: Use dedicated slab buckets for memdup

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Date: Mon,  4 Mar 2024 10:49:32 -0800
From: Kees Cook <keescook@...omium.org>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm@...ck.org,
	"GONG, Ruiqi" <gongruiqi@...weicloud.com>,
	Xiu Jianfeng <xiujianfeng@...wei.com>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Hyeonggon Yoo <42.hyeyoo@...il.com>,
	Christian Brauner <brauner@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Jan Kara <jack@...e.cz>,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: [PATCH 4/4] mm/util: Use dedicated slab buckets for memdup_user()

The prctl() PR_SET_VMA_ANON_NAME command can be used for exploiting[1]
use-after-free type confusion flaws in the kernel. This is just one
path to memdup_user() which is designed for contents coming from
userspace. Avoid having a user-controlled size cache share the global
kmalloc allocator by using a separate set of kmalloc buckets.

After a fresh boot under Ubuntu 23.10, we can see the caches are already
in use:

 # grep ^memdup /proc/slabinfo
 memdup_user-8k         4      4   8192    4    8 : ...
 memdup_user-4k         0      0   4096    8    8 : ...
 memdup_user-2k        16     16   2048   16    8 : ...
 memdup_user-1k         0      0   1024   16    4 : ...
 memdup_user-512        0      0    512   16    2 : ...
 memdup_user-256        0      0    256   16    1 : ...
 memdup_user-128        0      0    128   32    1 : ...
 memdup_user-64       256    256     64   64    1 : ...
 memdup_user-32       512    512     32  128    1 : ...
 memdup_user-16      1024   1024     16  256    1 : ...
 memdup_user-8       2048   2048      8  512    1 : ...
 memdup_user-192        0      0    192   21    1 : ...
 memdup_user-96       168    168     96   42    1 : ...

Link: https://starlabs.sg/blog/2023/07-prctl-anon_vma_name-an-amusing-heap-spray/ [1]
Signed-off-by: Kees Cook <keescook@...omium.org>
---
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org
---
 mm/util.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/mm/util.c b/mm/util.c
index 5a6a9802583b..818e74d11fb6 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -181,6 +181,16 @@ char *kmemdup_nul(const char *s, size_t len, gfp_t gfp)
 }
 EXPORT_SYMBOL(kmemdup_nul);
 
+static struct kmem_buckets *user_buckets __ro_after_init;
+
+static int __init init_user_buckets(void)
+{
+	user_buckets = kmem_buckets_create("memdup_user", 0, 0, 0, UINT_MAX, NULL);
+
+	return 0;
+}
+subsys_initcall(init_user_buckets);
+
 /**
  * memdup_user - duplicate memory region from user space
  *
@@ -194,7 +204,7 @@ void *memdup_user(const void __user *src, size_t len)
 {
 	void *p;
 
-	p = kmalloc_track_caller(len, GFP_USER | __GFP_NOWARN);
+	p = kmem_buckets_alloc(user_buckets, len, GFP_USER | __GFP_NOWARN);
 	if (!p)
 		return ERR_PTR(-ENOMEM);
 
-- 
2.34.1