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[83.28.45.75]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-5a4187e11ecsm3934256e87.47.2026.04.21.13.05.51 (version=TLS1_2 cipher=AES128-SHA bits=128/128); Tue, 21 Apr 2026 13:05:52 -0700 (PDT) Date: Tue, 21 Apr 2026 22:05:47 +0200 From: Michal Pecio To: Morduan Zang Cc: petkan@nucleusys.com, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, andrew+netdev@lunn.ch, linux-usb@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, Zhan Jun , syzbot+3f46c095ac0ca048cb71@syzkaller.appspotmail.com Subject: Re: [PATCH] net: usb: rtl8150: fix use-after-free in rtl8150_start_xmit() Message-ID: <20260421220547.2f7f9d85.michal.pecio@gmail.com> In-Reply-To: <73ACB7391A6DE033+20260421110412.14795-1-zhangdandan@uniontech.com> References: <73ACB7391A6DE033+20260421110412.14795-1-zhangdandan@uniontech.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Tue, 21 Apr 2026 19:04:12 +0800, Morduan Zang wrote: > From: Zhan Jun > > syzbot reported a KASAN slab-use-after-free read in rtl8150_start_xmit() > when accessing skb->len for tx statistics after usb_submit_urb() has > been called: > > BUG: KASAN: slab-use-after-free in rtl8150_start_xmit+0x71f/0x760 > drivers/net/usb/rtl8150.c:712 > Read of size 4 at addr ffff88810eb7a930 by task kworker/0:4/5226 > > The URB completion handler write_bulk_callback() frees the skb via > dev_kfree_skb_irq(dev->tx_skb). The URB may complete on another CPU > in softirq context before usb_submit_urb() returns in the submitter, > so by the time the submitter reads skb->len the skb has already been > queued to the per-CPU completion_queue and freed by net_tx_action(): > > CPU A (xmit) CPU B (USB completion softirq) > ------------ ------------------------------ > dev->tx_skb = skb; > usb_submit_urb() --+ > |-------> write_bulk_callback() > | dev_kfree_skb_irq(dev->tx_skb) > | net_tx_action() > | napi_skb_cache_put() <-- free > netdev->stats.tx_bytes | > += skb->len; <-- UAF read > > Fix it by caching skb->len before submitting the URB and using the > cached value when updating the tx_bytes counter. Question: Is it correct that ETH_ZLEN padding isn't counted in tx_bytes? > This mirrors the fix pattern used by other USB network drivers. Which ones? I looked at a few and they either: - appear to have the same bug (kaweth) - update stats on URB completion, right before freeing skb - copy data out of skb, update stats, free skb before URB completion Regards, Michal