From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 705111091916 for ; Thu, 19 Mar 2026 22:00:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=0yp23tNbPW94JMNdRGMZBEkiUIPkf8ue7dSq7oobTEQ=; b=bueo6cRF1bM7wG9aXtPTAq3ivY 5V7W2qHZOF9KM3AGtIik/XAwgdNP7Yo8+XkM/sm9UxgTLMN4xR3xDJE0igotzRWr6RwHUGJf5DCPy j5QQYemFs4RVtEeXH8ZBjc9cN7/6ZoLS3tffAZ+OlLAVnGNulWrFlYXfSP0zb+0xKDElnvHWiKihf Xay7JGxsdBSsNSZYtpUY2DiVeb5wSc16MRbzC4uQlN/RxebiYIlHxdBQgTxed+4GjOADG3rugoOU1 9+VBh/AGY79XGh3TGjiZLvEaisyIVxLhW97VaRv5rX+2tB9gQtpTUf+kfLOmHfZNQlntwWL+EmDxI z1CtyHuA==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1w3LPK-0000000Bc2m-0whL; Thu, 19 Mar 2026 21:59:58 +0000 Received: from pandora.armlinux.org.uk ([2001:4d48:ad52:32c8:5054:ff:fe00:142]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1w3LPG-0000000Bc1i-3H6t for linux-arm-kernel@lists.infradead.org; Thu, 19 Mar 2026 21:59:56 +0000 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=armlinux.org.uk; s=pandora-2019; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=0yp23tNbPW94JMNdRGMZBEkiUIPkf8ue7dSq7oobTEQ=; b=weV3nqI7chBqru/dlHBoOpFFn3 xshJScE+Z/b4faKZyekCELpOCgC109DFD4+6JYL6xn/mVHHv5AQ1Q6hYtr3o8Ju7ZVH28Gk3uRa5+ voQiCnmMiErPZfckGZhef6YsihnyV13ZoybKYjvTG8a2NDOPgvdfeiuA7sKx6DGOVzWb14LTKnzxo VHQiIyorzK6RcDnFuC7Lgr/tYhk0RZ7NfngmXG3X86idQmG+wfpLArb58r514rquGwA5KwhmUI7Li UCKgfb3oDY8bKXDGd2xhZbuNZP7ns8jZJ3NUY4Tzo9FuyEhFYcMXpp1YbeCK7o0iV0dehkagz6tLl rdyOkYkg==; Received: from shell.armlinux.org.uk ([fd8f:7570:feb6:1:5054:ff:fe00:4ec]:60868) by pandora.armlinux.org.uk with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.98.2) (envelope-from ) id 1w3LP2-000000005Mx-2VQf; Thu, 19 Mar 2026 21:59:40 +0000 Received: from linux by shell.armlinux.org.uk with local (Exim 4.98.2) (envelope-from ) id 1w3LOy-0000000008l-2rCx; Thu, 19 Mar 2026 21:59:36 +0000 Date: Thu, 19 Mar 2026 21:59:36 +0000 From: "Russell King (Oracle)" To: Sam Edwards Cc: Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Maxime Chevallier , Ovidiu Panait , Vladimir Oltean , Baruch Siach , Serge Semin , netdev@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH net v2 1/2] net: stmmac: Prevent NULL deref when RX memory exhausted Message-ID: References: <20260319184031.8596-1-CFSworks@gmail.com> <20260319184031.8596-2-CFSworks@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260319184031.8596-2-CFSworks@gmail.com> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260319_145955_105405_11999447 X-CRM114-Status: GOOD ( 24.02 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Mar 19, 2026 at 11:40:30AM -0700, Sam Edwards wrote: > The CPU receives frames from the MAC through conventional DMA: the CPU > allocates buffers for the MAC, then the MAC fills them and returns > ownership to the CPU. For each hardware RX queue, the CPU and MAC > coordinate through a shared ring array of DMA descriptors: one > descriptor per DMA buffer. Each descriptor includes the buffer's > physical address and a status flag ("OWN") indicating which side owns > the buffer: OWN=0 for CPU, OWN=1 for MAC. The CPU is only allowed to set > the flag and the MAC is only allowed to clear it, and both must move > through the ring in sequence: thus the ring is used for both > "submissions" and "completions." > > In the stmmac driver, stmmac_rx() bookmarks its position in the ring > with the `cur_rx` index. The main receive loop in that function checks > for rx_descs[cur_rx].own=0, gives the corresponding buffer to the > network stack (NULLing the pointer), and increments `cur_rx` modulo the > ring size. After the loop exits, stmmac_rx_refill(), which bookmarks its > position with `dirty_rx`, allocates fresh buffers and rearms the > descriptors (setting OWN=1). If it fails any allocation, it simply stops > early (leaving OWN=0) and will retry where it left off when next called. > > This means descriptors have a three-stage lifecycle (terms my own): > - `empty` (OWN=1, buffer valid) > - `full` (OWN=0, buffer valid and populated) > - `dirty` (OWN=0, buffer NULL) > > But because stmmac_rx() only checks OWN, it confuses `full`/`dirty`. In > the past (see 'Fixes:'), there was a bug where the loop could cycle > `cur_rx` all the way back to the first descriptor it dirtied, resulting > in a NULL dereference when mistaken for `full`. The aforementioned > commit resolved that *specific* failure by capping the loop's iteration > limit at `dma_rx_size - 1`, but this is only a partial fix: if the > previous stmmac_rx_refill() didn't complete, then there are leftover > `dirty` descriptors that the loop might encounter without needing to > cycle fully around. The current code therefore panics (see 'Closes:') > when stmmac_rx_refill() is memory-starved long enough for `cur_rx` to > catch up to `dirty_rx`. > > Fix this by further tightening the clamp from `dma_rx_size - 1` to > `dma_rx_size - stmmac_rx_dirty() - 1`, subtracting any remnant dirty > entries and limiting the loop so that `cur_rx` cannot catch back up to > `dirty_rx`. This carries no risk of arithmetic underflow: since the > maximum possible return value of stmmac_rx_dirty() is `dma_rx_size - 1`, > the worst the clamp can do is prevent the loop from running at all. Isn't the limit equivalent to: space = CIRC_SPACE(rx_q->cur_rx, rx_q->dirty_rx, priv->dma_conf.dma_rx_size); limit = min_t(unsigned int, space, limit); (Think of the "full" and "empty" cases as "space" which can be consumed to provide entries for the refiller to action.) I think the same applies for patch 2 - when the above returns zero it means we have no entries in the ring that aren't due for refill. Have you checked whether stmmac_rx_zc() also buggy in this respect? -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!