From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-ot1-f66.google.com (mail-ot1-f66.google.com [209.85.210.66]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5823F3C5538 for ; Mon, 20 Apr 2026 19:34:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.210.66 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776713676; cv=none; b=aIpnmdNak4nk1Hg0f10ZNeFOQVoHDMG+biu/gwATNg9ui0N2/AqUSJ3DIyLk5AGq0rAVquZQmLMO1Y4Z2hwrFUeByTIGLkR2/TaiVwKiww6bxq32DoYD6uhb4xS/+hHO+TDpWGOG0j6opNzUa73xA9bxr9oI79M6m7+Zhng58fs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776713676; c=relaxed/simple; bh=JSsKbyP1KNpH3VgRE1/5fzA0f6PCalqc1PPHjq93CEE=; h=Date:Message-ID:From:To:Cc:In-Reply-To:Subject:Content-Type; b=NHMD1v9vs0g5vC7WgI96mU3Sj5IFT+EcfA1mTDBfk2MBeGqGQu8Yx3yKzRUgNrlJvPG+hmLRRKhvedvl+IGH4Pr74sDCOsq/3ljKZUGFi/Skh4Xf7KT7f9jBPgogOkfiFYnzhzdMvFNDT0Zp9+NCkLm/cJHldVtbVBcr2jnQVkw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=VKoNcu/4; arc=none smtp.client-ip=209.85.210.66 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="VKoNcu/4" Received: by mail-ot1-f66.google.com with SMTP id 46e09a7af769-7dbd23bc684so2052294a34.2 for ; Mon, 20 Apr 2026 12:34:35 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20251104; t=1776713674; x=1777318474; darn=vger.kernel.org; h=subject:in-reply-to:cc:to:from:message-id:date:from:to:cc:subject :date:message-id:reply-to; bh=/N6MVvai13qDwjLjyRX2yoaXM7uyr51YiE4c9y/nXV0=; b=VKoNcu/4jsmvBtOmsntBF9EbRwgMMuhysTUj+lIB7jcHEbbiWGsWIXwt+lMEbHKXoq +RD/Ah6RPZi+9WBueZW15HclScJXfEw6pIvxMGE3qZ1mNvQ0SRlbJBEyHXwi82glzeas N3N0wzd/FC4k6INEAG1Tju91lGJEOHx23LnOn9vcgksxfmz5T0lOoA5gAuyW1HG3lF89 8GGmdYAjY/4u/bKsJ9HjqWbSE8i11rdN5XM0ZEc1fXuTMAZPFnfTzDTCPfNJoZbTKSK4 56YeHqreEFj4a9Q8Aps8OrlanfvKWKNGizj42lGtcxK7DRCXway//dO2DyebsqB9h22A iwag== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1776713674; x=1777318474; h=subject:in-reply-to:cc:to:from:message-id:date:x-gm-gg :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=/N6MVvai13qDwjLjyRX2yoaXM7uyr51YiE4c9y/nXV0=; b=JJup1B7RMmGWpAWXSjbkf0zqc7qyZATAgwo8gx2c57R6GkQNK+Dfjdqr9ADPGI6IUb /DfRAAaoWfbqDhs7/vUPNUHAQ7o73/T5yaXVc719UKBSsxDVTriim3b7q3l9Zc1rZ5Mg OqrHOdxffLvQdCYxnqTxQbUcp2BNN0DVElv/p7rYyAb6pS3g0F4l6vCSUkmBHdDVL6jm qFbpJsEoJfpHeSNWk6VCTBeDr9hdvXuuFCG0fP+JZLFRt+uy31F+5hMYipQ3vguau2XH EVOo9diolewqNgNfevn/n8/NXDQ4g49ekPF4ZNX2pCxvDCVsAWnGbO/v2aRemZ2WRnVm WO8Q== X-Gm-Message-State: AOJu0Yz7IYshYluX38DHLUXHc47Xq9ivhXhc/8b4HXTgBmcz5XZ6h2V/ sTmoj5nkdhUctGSropfKIr4WQBsFesrx2NI3HV/wuZ2IMHuEDiX0d00b X-Gm-Gg: AeBDievS70t8vw5E3QRpZrMSLXD81F436urBd1gbNpUPkdUAS+5Um96X06UUl5eKLFR MS0rlAQ1mQid68f8sCRlVFZfNC4944C/jmACs3YXsiwe5pAVXkKEDlnYsWUTf+CVOs5Z/BZuTkw eh1wdg+jsrnpYmhuABvywngGnrr/DP7/7ctBml7HC76gXlbFPVT1eXR85UBKO7LFo+mibfYyEpo hrGMuPbUoTA09YZ1vEiVGrWPurndqu3nrj05T+5PIeSAENB7QcggfAy0mCFVGShyNDdBL3K3Zy4 qpICwKFHJx7r/zAOaPu4Ppdc/Rifl7W8kSV8hoLAffR26bHJ056xRKSNBw8m6t5W08cIpCaGC/s XYERl5ZdIYoJBh0JzY85jVQ0h4QrEn450WIbELc+SuP60U7xRStOM1eAAa+KBW5ud0i40HUQuZy lOntPl20f6TcuJKaeHvwaMaO/Vvjxh X-Received: by 2002:a05:6830:6af8:b0:7d7:da02:1aa4 with SMTP id 46e09a7af769-7dc94fac1d6mr8621768a34.1.1776713674272; Mon, 20 Apr 2026 12:34:34 -0700 (PDT) Received: from localhost ([2a03:2880:12ff:71::]) by smtp.gmail.com with ESMTPSA id 46e09a7af769-7dc97518b58sm9283895a34.7.2026.04.20.12.34.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 20 Apr 2026 12:34:33 -0700 (PDT) Date: Mon, 20 Apr 2026 12:34:32 -0700 Message-ID: From: Stanislav Fomichev To: Jason Xing Cc: bpf@vger.kernel.org, netdev@vger.kernel.org, Jason Xing In-Reply-To: <20260420082805.14844-4-kerneljasonxing@gmail.com> Subject: Re: [PATCH net v2 3/8] xsk: fix use-after-free of xs->skb in xsk_build_skb() free_err path Content-Type: text/plain Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: > From: Jason Xing > > When xsk_build_skb() processes multi-buffer packets in copy mode, the > first descriptor stores data into the skb linear area without adding > any frags, so nr_frags stays at 0. The caller then sets xs->skb = skb > to accumulate subsequent descriptors. > > If a continuation descriptor fails (e.g. alloc_page returns NULL with > -EAGAIN), we jump to free_err where the condition: > > if (skb && !skb_shinfo(skb)->nr_frags) > kfree_skb(skb); > > evaluates to true because nr_frags is still 0 (the first descriptor > used the linear area, not frags). This frees the skb while xs->skb > still points to it, creating a dangling pointer. On the next transmit > attempt or socket close, xs->skb is dereferenced, causing a > use-after-free or double-free. > > Fix by adding a !xs->skb check to the condition, ensuring we only free > skbs that were freshly allocated in this call (xs->skb is NULL) and > never free an in-progress multi-buffer skb that the caller still > references. > > Closes: https://lore.kernel.org/all/20260415082654.21026-4-kerneljasonxing@gmail.com/ > Fixes: 6b9c129c2f93 ("xsk: remove @first_frag from xsk_build_skb()") > Signed-off-by: Jason Xing > --- > net/xdp/xsk.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c > index 6521604f8d42..4fdd1a45a9bd 100644 > --- a/net/xdp/xsk.c > +++ b/net/xdp/xsk.c > @@ -889,7 +889,7 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs, > return skb; > > free_err: > - if (skb && !skb_shinfo(skb)->nr_frags) > + if (skb && !xs->skb && !skb_shinfo(skb)->nr_frags) > kfree_skb(skb); > > if (err == -EOVERFLOW) { > -- > 2.41.3 Now "!skb_shinfo(skb)->nr_frags" feels redundant? It's either "skb && !xs->skb" and we own the kfree. or "xs->skb != NULL" and we want xsk_drop_skb? Or am I missing something?