From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from pandora.armlinux.org.uk (pandora.armlinux.org.uk [78.32.30.218]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DB99B402BB2; Thu, 19 Mar 2026 21:59:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=78.32.30.218 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773957593; cv=none; b=NqQHWU7sibXBN6xAJqa/IO0PRKCASykZNgzsAdjRclXIoNIs6rwi42yf2u03yrgE2zY5OEUrqoapdUnYzwpBpXcBwNJwK40M1WYlyXUbwRkiWXP8OwnIYzFWzLpljvcL9ViD8u22T5eGueAFuEacYe6uewL1SBQjFGW9PK1m3Yw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773957593; c=relaxed/simple; bh=ZSq1KbVoUB+K1at8AD5hG1nGBkwED9vYe5QiDWUG7Xc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=tEprwBWsRbKI2yJzoFiqCnggV0WrgZMsHrcwdh10xKUr9BGL0IWdeHwfgQTko9t54gq5207TM77uxvse7venqyIuqLq5G5K6oyNXRRtQbqnUwP5uYPmwV6pzFxFetEDPiy6PBAE2bI0BNZ0gnG9c5MZNdXFDplVGt0kZ6se6Nl0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=armlinux.org.uk; spf=none smtp.mailfrom=armlinux.org.uk; dkim=pass (2048-bit key) header.d=armlinux.org.uk header.i=@armlinux.org.uk header.b=weV3nqI7; arc=none smtp.client-ip=78.32.30.218 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=armlinux.org.uk Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=armlinux.org.uk Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=armlinux.org.uk header.i=@armlinux.org.uk header.b="weV3nqI7" 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> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260319184031.8596-2-CFSworks@gmail.com> Sender: Russell King (Oracle) 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!