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Fri, 15 May 2026 02:24:03 -0700 (PDT) Received: from [192.168.88.32] ([212.105.155.231]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48fe57944c1sm56922025e9.7.2026.05.15.02.24.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 15 May 2026 02:24:03 -0700 (PDT) Message-ID: <36ffcd43-ebda-44d8-9e32-4268826deb49@redhat.com> Date: Fri, 15 May 2026 11:24:01 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net] net/stmmac: Fix free-after-use panic when interface goes does with XDP To: Jakub Raczynski Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, andrew+netdev@lunn.ch, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, mcoquelin.stm32@gmail.com, alexandre.torgue@foss.st.com, k.domagalski@samsung.com, k.tegowski@samsung.com, Chang-Sub Lee References: <20260511165045.3091475-1-j.raczynski@samsung.com> <4ca19ae1-6606-433b-9860-0a8cce12f80f@redhat.com> From: Paolo Abeni Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 5/15/26 10:29 AM, Jakub Raczynski wrote: > On Thu, May 14, 2026 at 02:01:20PM +0200, Paolo Abeni wrote: >>> >>> Fix this by following: >>> - Set STMMAC_DOWN flag before stopping DMA to signal XDP to stop >>> - Call synchronize_rcu() after stopping DMA but before freeing resources to >>> ensure all ongoing NAPI operations complete >>> - Add STMMAC_DOWN flag checks in XDP code paths (XDP_TX and XDP_REDIRECT) to >>> drop packets when interface is going down. This has already been done for >>> stmmac_xdp_xmit() so make it consistent >>> - Clear STMMAC_DOWN flag in __stmmac_open() to restore normal operation. >>> This was only done for stmmac_reset_subtask() during abnormal operation, >>> which is not enough. This does not affect normal operation as this flag is >>> used only for XDP apps >> >> The above looks racy. I think instead you should just use >> napi_synchronize() in __stmmac_release. >> > You put this after whole section, but I assume you are talking about > synchronize_rcu()? Because currently there are 0 checks and it is pure race > condition. synchronize_rcu() does secure it in some way, but you are correct, > proper ensuring that napi has finished is napi_synchronize(). > Will fix in v2. >> >>> @@ -5267,6 +5279,9 @@ static int stmmac_xdp_xmit_back(struct stmmac_priv *priv, >>> if (unlikely(!xdpf)) >>> return STMMAC_XDP_CONSUMED; >>> >>> + if (unlikely(test_bit(STMMAC_DOWN, &priv->state))) >>> + return -ENETDOWN; >> >> Sashiko noted here you should return STMMAC_XDP_CONSUMED >> >> /P > Seems good, will fix in v2. If you use napi_synchronize(), I think you can avoid setting STMMAC_DOWN and testing it in the fast path: the run-to-completion after irq disabling should ensure that no tx could happen after that napi_synchronize() completes. Side note: it's not clear to me where/when irq disabling take place?!? /P