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Message-ID: <CANn89iLfPmEW6-sSgZXXFn5H72xc_5H4w+qvA5dMjcARBB7v9A@mail.gmail.com>
Date: Tue, 23 Dec 2025 11:59:23 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: bestswngs@...il.com, security@...nel.org, davem@...emloft.net,
kuba@...nel.org, horms@...nel.org, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org, xmei5@....edu
Subject: Re: [PATCH net v4] net: skbuff: add usercopy region to skbuff_fclone_cache
On Tue, Dec 23, 2025 at 11:34 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On 12/16/25 9:44 AM, 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
> >
> > 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>
>
> Rephrasing Eric's comment (and hoping to have not misread it), you
> should fix the issue differently, catching and fclones before adding
> them to the error queue and try to unclone them.
Instead of opening/weakening skbuff_clone to wide user copies, I would rather
use what we did in:
commit 2558b8039d059342197610498c8749ad294adee5
Author: Eric Dumazet <edumazet@...gle.com>
Date: Mon Feb 13 16:00:59 2023 +0000
net: use a bounce buffer for copying skb->mark
ie :
diff --git a/net/core/sock.c b/net/core/sock.c
index 45c98bf524b2..a1c8b47b0d56 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -3896,7 +3896,7 @@ void sock_enable_timestamp(struct sock *sk, enum
sock_flags flag)
int sock_recv_errqueue(struct sock *sk, struct msghdr *msg, int len,
int level, int type)
{
- struct sock_exterr_skb *serr;
+ struct sock_extended_err ee;
struct sk_buff *skb;
int copied, err;
@@ -3916,8 +3916,9 @@ int sock_recv_errqueue(struct sock *sk, struct
msghdr *msg, int len,
sock_recv_timestamp(msg, sk, skb);
- serr = SKB_EXT_ERR(skb);
- put_cmsg(msg, level, type, sizeof(serr->ee), &serr->ee);
+ /* We must use a bounce buffer for CONFIG_HARDENED_USERCOPY=y */
+ ee = SKB_EXT_ERR(skb)->ee;
+ put_cmsg(msg, level, type, sizeof(ee), &ee);
msg->msg_flags |= MSG_ERRQUEUE;
err = copied;
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