Re: [PATCH net v5] net: sock: fix hardened usercopy panic in sock_recv_errqueue

[Weiming Shi <bestswngs@gmail.com>]

On 25-12-24 09:52, Eric Dumazet wrote:
> On Tue, Dec 23, 2025 at 9:36 PM <bestswngs@gmail.com> wrote:
> >
> > From: Weiming Shi <bestswngs@gmail.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@asu.edu>
> > Signed-off-by: Weiming Shi <bestswngs@gmail.com>
> > ---
> 
> Reviewed-by: Eric Dumazet <edumazet@google.com>
> 
> Also please next time do not CC security@kernel.org if you made the
> bug public (say on netdev@kernel.org),
> because security@kernel.org will be of no help.
> 
> Congratulations on your first linux contribution !

Hi Eric,

Thank you very much for your quick review and encouragement.
I will take note of your suggestions next time.

Best regards,
Weiming