net: skbuff: fix usercopy violation in skbuff_fclone_cache

net: skbuff: add usercopy region to skbuff_fclone_cache

skbuff_fclone_cache was created without defining a usercopy region,
unlike skbuff_head_cache which properly whitelists the cb[] field.
This causes a usercopy BUG() when CONFIG_HARDENED_USERCOPY is enabled
and the kernel attempts to copy sk_buff.cb data to userspace via
sock_recv_errqueue() -> put_cmsg().

The crash occurs when:
1. TCP allocates an skb using alloc_skb_fclone() (from skbuff_fclone_cache)
2. The skb is cloned via skb_clone() using the pre-allocated fclone
3. The cloned skb is queued to sk_error_queue for timestamp reporting
4. Userspace reads the error queue via recvmsg(MSG_ERRQUEUE)
5. sock_recv_errqueue() calls put_cmsg() to copy serr->ee from skb->cb
6. __check_heap_object() fails because skbuff_fclone_cache has no
   usercopy whitelist

When cloned skbs allocated from skbuff_fclone_cache are used in the
socket error queue, accessing the sock_exterr_skb structure in skb->cb
via put_cmsg() triggers a usercopy hardening violation:

[    5.379589] usercopy: Kernel memory exposure attempt detected from SLUB object 'skbuff_fclone_cache' (offset 296, size 16)!
[    5.382796] kernel BUG at mm/usercopy.c:102!
[    5.383923] Oops: invalid opcode: 0000 [kernel-patches#1] SMP KASAN NOPTI
[    5.384903] CPU: 1 UID: 0 PID: 138 Comm: poc_put_cmsg Not tainted 6.12.57 kernel-patches#7
[    5.384903] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
[    5.384903] RIP: 0010:usercopy_abort+0x6c/0x80
[    5.384903] Code: 1a 86 51 48 c7 c2 40 15 1a 86 41 52 48 c7 c7 c0 15 1a 86 48 0f 45 d6 48 c7 c6 80 15 1a 86 48 89 c1 49 0f 45 f3 e8 84 27 88 ff <0f> 0b 490
[    5.384903] RSP: 0018:ffffc900006f77a8 EFLAGS: 00010246
[    5.384903] RAX: 000000000000006f RBX: ffff88800f0ad2a8 RCX: 1ffffffff0f72e74
[    5.384903] RDX: 0000000000000000 RSI: 0000000000000004 RDI: ffffffff87b973a0
[    5.384903] RBP: 0000000000000010 R08: 0000000000000000 R09: fffffbfff0f72e74
[    5.384903] R10: 0000000000000003 R11: 79706f6372657375 R12: 0000000000000001
[    5.384903] R13: ffff88800f0ad2b8 R14: ffffea00003c2b40 R15: ffffea00003c2b00
[    5.384903] FS:  0000000011bc4380(0000) GS:ffff8880bf100000(0000) knlGS:0000000000000000
[    5.384903] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    5.384903] CR2: 000056aa3b8e5fe4 CR3: 000000000ea26004 CR4: 0000000000770ef0
[    5.384903] PKRU: 55555554
[    5.384903] Call Trace:
[    5.384903]  <TASK>
[    5.384903]  __check_heap_object+0x9a/0xd0
[    5.384903]  __check_object_size+0x46c/0x690
[    5.384903]  put_cmsg+0x129/0x5e0
[    5.384903]  sock_recv_errqueue+0x22f/0x380
[    5.384903]  tls_sw_recvmsg+0x7ed/0x1960
[    5.384903]  ? srso_alias_return_thunk+0x5/0xfbef5
[    5.384903]  ? schedule+0x6d/0x270
[    5.384903]  ? srso_alias_return_thunk+0x5/0xfbef5
[    5.384903]  ? mutex_unlock+0x81/0xd0
[    5.384903]  ? __pfx_mutex_unlock+0x10/0x10
[    5.384903]  ? __pfx_tls_sw_recvmsg+0x10/0x10
[    5.384903]  ? _raw_spin_lock_irqsave+0x8f/0xf0
[    5.384903]  ? _raw_read_unlock_irqrestore+0x20/0x40
[    5.384903]  ? srso_alias_return_thunk+0x5/0xfbef5

Fix by using kmem_cache_create_usercopy() with the same cb[] region
whitelist as skbuff_head_cache.

Signed-off-by: weiming shi (Swing) <bestswngs@gmail.com>
Reported-by: Xiang Mei xmei5@asu.edu
Reported-by: weiming shi (Swing) bestswngs@gmail.com
Signed-off-by: NipaLocal <nipa@local>

Author: WinMin

net/core/skbuff.c

@@ -622,7 +622,7 @@ static void *kmalloc_reserve(unsigned int *size, gfp_t flags, int node,
 	return obj;
 }
 
-/* 	Allocate a new skbuff. We do this ourselves so we can fill in a few
+/*	Allocate a new skbuff. We do this ourselves so we can fill in a few
  *	'private' fields and also do memory statistics to find all the
  *	[BEEP] leaks.
  *
@@ -5157,11 +5157,14 @@ void __init skb_init(void)
 					      NULL);
 	skbuff_cache_size = kmem_cache_size(net_hotdata.skbuff_cache);
 
-	net_hotdata.skbuff_fclone_cache = kmem_cache_create("skbuff_fclone_cache",
-						sizeof(struct sk_buff_fclones),
-						0,
-						SLAB_HWCACHE_ALIGN|SLAB_PANIC,
-						NULL);
+	net_hotdata.skbuff_fclone_cache = kmem_cache_create_usercopy("skbuff_fclone_cache",
+		sizeof(struct sk_buff_fclones),
+		0,
+		SLAB_HWCACHE_ALIGN | SLAB_PANIC,
+		offsetof(struct sk_buff, cb),
+		sizeof(struct sk_buff) + sizeof_field(struct sk_buff, cb),
+		NULL);
+
 	/* usercopy should only access first SKB_SMALL_HEAD_HEADROOM bytes.
 	 * struct skb_shared_info is located at the end of skb->head,
 	 * and should not be copied to/from user.