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Mon, 15 Dec 2025 10:31:25 -0800 (PST) Received: from SLSGDTSWING002.tail0ac356.ts.net ([129.126.109.177]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-29f25cb1609sm106467285ad.57.2025.12.15.10.31.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 15 Dec 2025 10:31:24 -0800 (PST) From: bestswngs@gmail.com To: security@kernel.org Cc: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, xmei5@asu.edu, "Weiming Shi" Subject: [PATCH v3] net: skbuff: add usercopy region to skbuff_fclone_cache Date: Tue, 16 Dec 2025 02:29:14 +0800 Message-ID: <20251215182913.955478-2-bestswngs@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" From: "Weiming Shi" 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()=20 (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 6.12.5= 7 #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: 1ffffffff0f= 72e74 [ 5.384903] RDX: 0000000000000000 RSI: 0000000000000004 RDI: ffffffff87b= 973a0 [ 5.384903] RBP: 0000000000000010 R08: 0000000000000000 R09: fffffbfff0f= 72e74 [ 5.384903] R10: 0000000000000003 R11: 79706f6372657375 R12: 00000000000= 00001 [ 5.384903] R13: ffff88800f0ad2b8 R14: ffffea00003c2b40 R15: ffffea00003= c2b00 [ 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: 00000000007= 70ef0 [ 5.384903] PKRU: 55555554 [ 5.384903] Call Trace: [ 5.384903] [ 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 In our patch, we referenced=20 net: Whitelist the `skb_head_cache` "cb" field. [5] 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=3D79a8a642bf05c Fixes: 6d07d1cd300f ("usercopy: Restrict non-usercopy caches to size 0") Reported-by: Xiang Mei Signed-off-by: Weiming Shi --- v2: Fix the Commit Message v3: Add "From" email adress, Fix "CC" and "TO" email address net/core/skbuff.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/net/core/skbuff.c b/net/core/skbuff.c index c52e955dd3a0..89c98ce6106a 100644 --- a/net/core/skbuff.c +++ b/net/core/skbuff.c @@ -5157,7 +5157,7 @@ void __init skb_init(void) NULL); skbuff_cache_size =3D kmem_cache_size(net_hotdata.skbuff_cache); =20 - net_hotdata.skbuff_fclone_cache =3D kmem_cache_create("skbuff_fclone_cach= e", + net_hotdata.skbuff_fclone_cache =3D kmem_cache_create_usercopy("skbuff_fc= lone_cache", sizeof(struct sk_buff_fclones), 0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, --=20 2.43.0