[PATCH 2/4] ipc, msg: Use dedicated slab buckets for alloc_msg()

[Kees Cook]

The msg subsystem is a common target for exploiting[1][2][3][4][5][6]
use-after-free type confusion flaws in the kernel for both read and
write primitives. 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 ^msg_msg /proc/slabinfo
 msg_msg-8k             0      0   8192    4    8 : ...
 msg_msg-4k            96    128   4096    8    8 : ...
 msg_msg-2k            64     64   2048   16    8 : ...
 msg_msg-1k            64     64   1024   16    4 : ...
 msg_msg-16          1024   1024     16  256    1 : ...
 msg_msg-8              0      0      8  512    1 : ...

Link: https://blog.hacktivesecurity.com/index.php/2022/06/13/linux-kernel-exploit-development-1day-case-study/ [1]
Link: https://hardenedvault.net/blog/2022-11-13-msg_msg-recon-mitigation-ved/ [2]
Link: https://www.willsroot.io/2021/08/corctf-2021-fire-of-salvation-writeup.html [3]
Link: https://a13xp0p0v.github.io/2021/02/09/CVE-2021-26708.html [4]
Link: https://google.github.io/security-research/pocs/linux/cve-2021-22555/writeup.html [5]
Link: https://zplin.me/papers/ELOISE.pdf [6]
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
---
 ipc/msgutil.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/ipc/msgutil.c b/ipc/msgutil.c
index d0a0e877cadd..36f1aa9ea1cf 100644
--- a/ipc/msgutil.c
+++ b/ipc/msgutil.c
@@ -42,6 +42,15 @@ struct msg_msgseg {
 #define DATALEN_MSG	((size_t)PAGE_SIZE-sizeof(struct msg_msg))
 #define DATALEN_SEG	((size_t)PAGE_SIZE-sizeof(struct msg_msgseg))
 
+static struct kmem_buckets *msg_buckets __ro_after_init;
+
+static int __init init_msg_buckets(void)
+{
+	msg_buckets = kmem_buckets_create("msg_msg", 0, SLAB_ACCOUNT, 0, 0, NULL);
+
+	return 0;
+}
+subsys_initcall(init_msg_buckets);
 
 static struct msg_msg *alloc_msg(size_t len)
 {
@@ -50,7 +59,7 @@ static struct msg_msg *alloc_msg(size_t len)
 	size_t alen;
 
 	alen = min(len, DATALEN_MSG);
-	msg = kmalloc(sizeof(*msg) + alen, GFP_KERNEL_ACCOUNT);
+	msg = kmem_buckets_alloc(msg_buckets, sizeof(*msg) + alen, GFP_KERNEL);
 	if (msg == NULL)
 		return NULL;
 
-- 
2.34.1