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When skb is queued to userspace it leaves softirq/rcu protection. skb->nfct (via conntrack extensions such as helper) could then reference modules that no longer exist if the conntrack was not yet confirmed. nf_ct_iterate_destroy() will set the DYING bit for unconfirmed conntracks, we therefore solve this race as follows: 1. take the queue spinlock. 2. check if the conntrack is unconfirmed and has dying bit set. In this case, we must discard skb while we're still inside rcu read-side section. 3. If nf_ct_iterate_destroy() is called right after the packet is queued to userspace, it will be removed from the queue via nf_ct_iterate_destroy -> nf_queue_nf_hook_drop. When userspace sends the verdict (nfnetlink takes rcu read lock), there are two cases to consider: 1. nf_ct_iterate_destroy() was called while packet was out. In this case, skb will have been removed from the queue already and no reinject takes place as we won't find a matching entry for the packet id. 2. nf_ct_iterate_destroy() gets called right after verdict callback found and removed the skb from queue list. In this case, skb->nfct is marked as dying but it is still valid. The skb will be dropped either in nf_conntrack_confirm (we don't insert DYING conntracks into hash table) or when we try to queue the skb again, but either events don't occur before the rcu read lock is dropped. Signed-off-by: Florian Westphal <fw@strlen.de> Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.