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Message-ID: <20251216084449.973244-5-bestswngs@gmail.com>
Date: Tue, 16 Dec 2025 16:44:53 +0800
From: bestswngs@...il.com
To: security@...nel.org
Cc: davem@...emloft.net,
edumazet@...gle.com,
kuba@...nel.org,
pabeni@...hat.com,
horms@...nel.org,
netdev@...r.kernel.org,
linux-kernel@...r.kernel.org,
xmei5@....edu,
Weiming Shi <bestswngs@...il.com>
Subject: [PATCH net v4] net: skbuff: add usercopy region to skbuff_fclone_cache
From: Weiming Shi <bestswngs@...il.com>
skbuff_fclone_cache was created without defining a usercopy region, [1]
unlike skbuff_head_cache which properly whitelists the cb[] field. [2]
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) [1]
2. The skb is cloned via skb_clone() using the pre-allocated fclone [3]
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 [4]
6. __check_heap_object() fails because skbuff_fclone_cache has no
usercopy whitelist [5]
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 [#1] SMP KASAN NOPTI
[ 5.384903] CPU: 1 UID: 0 PID: 138 Comm: poc_put_cmsg Not tainted 6.12.57 #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
The crash offset 296 corresponds to skb2->cb within skbuff_fclones:
- sizeof(struct sk_buff) = 232
- offsetof(struct sk_buff, cb) = 40
- offset of skb2.cb in fclones = 232 + 40 = 272
- crash offset 296 = 272 + 24 (inside sock_exterr_skb.ee)
Fix this by using kmem_cache_create_usercopy() for skbuff_fclone_cache
and whitelisting the cb regions.
In our patch, we referenced
net: Whitelist the `skb_head_cache` "cb" field. [6]
Fix by using kmem_cache_create_usercopy() with the same cb[] region
whitelist as skbuff_head_cache.
[1] https://elixir.bootlin.com/linux/v6.12.62/source/net/ipv4/tcp.c#L885
[2] https://elixir.bootlin.com/linux/v6.12.62/source/net/core/skbuff.c#L5104
[3] https://elixir.bootlin.com/linux/v6.12.62/source/net/core/skbuff.c#L5566
[4] https://elixir.bootlin.com/linux/v6.12.62/source/net/core/skbuff.c#L5491
[5] https://elixir.bootlin.com/linux/v6.12.62/source/mm/slub.c#L5719
[6] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=79a8a642bf05c
Fixes: 6d07d1cd300f ("usercopy: Restrict non-usercopy caches to size 0")
Reported-by: Xiang Mei <xmei5@....edu>
Signed-off-by: Weiming Shi <bestswngs@...il.com>
---
v2: Fix the Commit Message
v3: Add "From" email address, Fix "CC" and "TO" email address
v4: Fix The Patch Code
net/core/skbuff.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index a00808f7be6a..89c98ce6106a 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5157,10 +5157,12 @@ 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",
+ 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,
--
2.43.0
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