mm/util: Use dedicated slab buckets for memdup_user()

Both memdup_user() and vmemdup_user() handle allocations that are
regularly used for exploiting use-after-free type confusion flaws in
the kernel (e.g. prctl() PR_SET_VMA_ANON_NAME[1] and setxattr[2][3][4]
respectively).

Since both are designed for contents coming from userspace, it allows
for userspace-controlled allocation sizes. Use a dedicated set of kmalloc
buckets so these allocations do not share caches with the global kmalloc
buckets.

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

 # grep ^memdup /proc/slabinfo
 memdup_user-8k         4      4   8192    4    8 : ...
 memdup_user-4k         8      8   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]
Link: https://duasynt.com/blog/linux-kernel-heap-spray [2]
Link: https://etenal.me/archives/1336 [3]
Link: https://github.com/a13xp0p0v/kernel-hack-drill/blob/master/drill_exploit_uaf.c [4]
Signed-off-by: Kees Cook <kees@kernel.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

mm/util.c

@@ -198,6 +198,16 @@ char *kmemdup_nul(const char *s, size_t len, gfp_t gfp)
 }
 EXPORT_SYMBOL(kmemdup_nul);
 
+static kmem_buckets *user_buckets __ro_after_init;
+
+static int __init init_user_buckets(void)
+{
+	user_buckets = kmem_buckets_create("memdup_user", 0, 0, INT_MAX, NULL);
+
+	return 0;
+}
+subsys_initcall(init_user_buckets);
+
 /**
  * memdup_user - duplicate memory region from user space
  *
@@ -211,7 +221,7 @@ 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_track_caller(user_buckets, len, GFP_USER | __GFP_NOWARN);
 	if (!p)
 		return ERR_PTR(-ENOMEM);
 
@@ -237,7 +247,7 @@ void *vmemdup_user(const void __user *src, size_t len)
 {
 	void *p;
 
-	p = kvmalloc(len, GFP_USER);
+	p = kmem_buckets_valloc(user_buckets, len, GFP_USER);
 	if (!p)
 		return ERR_PTR(-ENOMEM);