On Thu, Aug 6, 2020 at 1:48 PM linmiaohe <[email protected]> wrote:
>
> From: Miaohe Lin <[email protected]>
>
> We could be trapped in deadloop when we try to copy userspace skb frags
> buffers to kernel with a cloned skb:
>
> [kbox] catch panic event, panic reason:kernel stack overflow
> [kbox] catch panic event, start logging.
> CPU: 3 PID: 4083 Comm: insmod Kdump: loaded Tainted: G OE 4.19 #6
> Hardware name: linux,dummy-virt (DT)
> Call trace:
> dump_backtrace+0x0/0x198
> show_stack+0x24/0x30
> dump_stack+0xa4/0xcc
> kbox_panic_notifier_callback+0x1d0/0x310 [kbox]
> notifier_call_chain+0x5c/0xa0
> atomic_notifier_call_chain+0x3c/0x50
> panic+0x164/0x314
> __stack_chk_fail+0x0/0x28
> handle_bad_stack+0xfc/0x108
> __bad_stack+0x90/0x94
> pskb_expand_head+0x0/0x2c8
> pskb_expand_head+0x290/0x2c8
> skb_copy_ubufs+0x3cc/0x520
> pskb_expand_head+0x290/0x2c8
> skb_copy_ubufs+0x3cc/0x520
> pskb_expand_head+0x290/0x2c8
> skb_copy_ubufs+0x3cc/0x520
> pskb_expand_head+0x290/0x2c8
> skb_copy_ubufs+0x3cc/0x520
> ...
> pskb_expand_head+0x290/0x2c8
> skb_copy_ubufs+0x3cc/0x520
> ...
>
> Reproduce code snippet:
> skb = alloc_skb(UBUF_DATA_LEN, GFP_ATOMIC);
> clone = skb_clone(skb, GFP_ATOMIC);
> skb_zcopy_set_nouarg(clone, NULL);
> pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
>
> Catch this unexpected case and return -EINVAL in skb_orphan_frags() before
> we call skb_copy_ubufs() to fix it.
Is this a hypothetical codepath?
skb zerocopy carefully tracks clone calls where necessary. See the
call to skb_orphan_frags in skb_clone, and the implementation of that
callee.
The only caller of skb zerocopy with nouarg is tpacket_fill_skb, as
of commit 5cd8d46ea156 ("packet: copy user buffers before orphan or
clone").
As the commit subject indicates, this sets skb_zcopy_set_nouarg
exactly to be sure that any clone will trigger a copy of "zerocopy"
user data to private kernel memory.
No clone must happen between alloc_skb and
skb_zcopy_set_nouarg, indeed. But AFAIK, none exists.
