Messages in this thread |  | | From | Eric Dumazet <> | | Date | Wed, 17 Dec 2025 19:24:07 +0100 | | Subject | Re: [PATCH net v4] net: skbuff: add usercopy region to skbuff_fclone_cache |
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On Tue, Dec 16, 2025 at 9:51 AM <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 > > 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@asu.edu> > Signed-off-by: Weiming Shi <bestswngs@gmail.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);
I have a bad feeling about this patch.
Really we should not put a fast clone skb back in error queue in the first place, because we can not control how long the (possibly large) skb will stay there.
Things like skb_fclone_busy() would need a fix otherwise.
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