linux-kernel - Re: [PATCH net v5] net: sock: fix hardened usercopy panic in sock_recv

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Message-ID: <CANn89iKpH0MO36JHbM=vS9ga=9UzgdtFahNR4+dvNr2oUtNLuA@mail.gmail.com>
Date: Wed, 24 Dec 2025 09:52:04 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: bestswngs@...il.com
Cc: security@...nel.org, davem@...emloft.net, kuba@...nel.org, 
	pabeni@...hat.com, horms@...nel.org, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, xmei5@....edu
Subject: Re: [PATCH net v5] net: sock: fix hardened usercopy panic in sock_recv_errqueue

On Tue, Dec 23, 2025 at 9:36 PM <bestswngs@...il.com> wrote:
>
> 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
>

Nit : Next time you send a stack trace, please run it though
scripts/decode_stacktrace.sh
to get meaningful symbols.

> 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)
>
> This patch uses a local stack variable as a bounce buffer to avoid the hardened usercopy check failure.
>
> [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
>
> Fixes: 6d07d1cd300f ("usercopy: Restrict non-usercopy caches to size 0")
> Reported-by: Xiang Mei <xmei5@....edu>
> Signed-off-by: Weiming Shi <bestswngs@...il.com>
> ---

Reviewed-by: Eric Dumazet <edumazet@...gle.com>

Also please next time do not CC security@...nel.org if you made the
bug public (say on netdev@...nel.org),
because security@...nel.org will be of no help.

Congratulations on your first linux contribution !