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[146.241.108.140]) by smtp.gmail.com with ESMTPSA id l5sm6579335wmh.0.2021.05.05.10.28.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 05 May 2021 10:28:09 -0700 (PDT) Message-ID: Subject: Re: [PATCH net 1/4] net: fix double-free on fraglist GSO skbs From: Paolo Abeni To: Willem de Bruijn Cc: Network Development , "David S. Miller" , Jakub Kicinski , Steffen Klassert , Willem de Bruijn , Miaohe Lin Date: Wed, 05 May 2021 19:28:08 +0200 In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.36.5 (3.36.5-2.fc32) MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Wed, 2021-05-05 at 12:13 -0400, Willem de Bruijn wrote: > On Wed, May 5, 2021 at 11:37 AM Paolo Abeni wrote: > > While segmenting a SKB_GSO_FRAGLIST GSO packet, if the destructor > > callback is available, the skb destructor is invoked on each > > aggregated packet via skb_release_head_state(). > > > > Such field (and the pairer skb->sk) is left untouched, so the same > > destructor is invoked again when the segmented skbs are freed, leading > > to double-free/UaF of the relevant socket. > > Similar to skb_segment, should the destructor be swapped with the last > segment and callback delayed, instead of called immediately as part of > segmentation? > > /* Following permits correct backpressure, for protocols > * using skb_set_owner_w(). > * Idea is to tranfert ownership from head_skb to last segment. > */ > if (head_skb->destructor == sock_wfree) { > swap(tail->truesize, head_skb->truesize); > swap(tail->destructor, head_skb->destructor); > swap(tail->sk, head_skb->sk); > } My understanding is that one assumption in the original SKB_GSO_FRAGLIST implementation was that SKB_GSO_FRAGLIST skbs are not owned by any socket. AFAICS the above assumption was true until: commit c75fb320d482a5ce6e522378d137fd2c3bf79225 Author: Paolo Abeni Date: Fri Apr 9 13:04:37 2021 +0200 veth: use skb_orphan_partial instead of skb_orphan after that, if the skb is owned, skb->destructor is sock_efree(), so the above code should not trigger. More importantly SKB_GSO_FRAGLIST can only be applied if the inner- most protocol is UDP, so commit 432c856fcf45c468fffe2e5029cb3f95c7dc9475 and d6a4a10411764cf1c3a5dad4f06c5ebe5194488b should not be relevant. Thanks! Paolo