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[23.234.115.121]) by smtp.gmail.com with UTF8SMTPSA id 586e51a60fabf-42fb269f69bsm2841483fac.12.2026.04.21.21.45.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 21 Apr 2026 21:45:09 -0700 (PDT) From: Sam Edwards X-Google-Original-From: Sam Edwards To: Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni Cc: Maxime Coquelin , Alexandre Torgue , "Russell King (Oracle)" , Maxime Chevallier , Ovidiu Panait , Vladimir Oltean , Baruch Siach , Serge Semin , Giuseppe Cavallaro , netdev@vger.kernel.org, linux-stm32@st-md-mailman.stormreply.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Sam Edwards , stable@vger.kernel.org, Russell King Subject: [PATCH net v6] net: stmmac: Prevent NULL deref when RX memory exhausted Date: Tue, 21 Apr 2026 21:45:03 -0700 Message-ID: <20260422044503.5349-1-CFSworks@gmail.com> X-Mailer: git-send-email 2.52.0 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 explicitly checking, before advancing `cur_rx`, if the next entry is dirty; exit the loop if so. This prevents processing of the final, used descriptor until stmmac_rx_refill() succeeds, but fully prevents the `cur_rx == dirty_rx` ambiguity as the previous bugfix intended: so remove the clamp as well. Since stmmac_rx_zc() is a copy-paste-and-tweak of stmmac_rx() and the code structure is identical, any fix to stmmac_rx() will also need a corresponding fix for stmmac_rx_zc(). Therefore, apply the same check there. In stmmac_rx() (not stmmac_rx_zc()), a related bug remains: after the MAC sets OWN=0 on the final descriptor, it will be unable to send any further DMA-complete IRQs until it's given more `empty` descriptors. Currently, the driver simply *hopes* that the next stmmac_rx_refill() succeeds, risking an indefinite stall of the receive process if not. But this is not a regression, so it can be addressed in a future change. Fixes: b6cb4541853c7 ("net: stmmac: avoid rx queue overrun") Closes: https://bugzilla.kernel.org/show_bug.cgi?id=221010 Cc: stable@vger.kernel.org Suggested-by: Russell King Signed-off-by: Sam Edwards --- This is v6 of [1], which was itself split out of [2]. This patch prevents a NULL dereference in the stmmac receive path, and (at Russell's suggestion) in the zero-copy path as well. The approach is different from the previous version and checks the dirty_rx index in the loop proper, copied directly from Russell's suggestion [3]. Parts of the commit message also use his phrasing. For these reasons he is credited with `Suggested-by`. The commit message now acknowledges the pipeline stall that can occur in case of failure of the next stmmac_rx_refill() after the MAC consumes the final descriptor. I still intend to fix that bug when I can find the time to finish investigating and implement the timer as requested by Jakub, however I'm sending this patch now to resolve the outright _panic_ and simplify review. The stmmac_rx_zc() path is not affected by this stall. [1] https://lore.kernel.org/netdev/20260415023947.7627-1-CFSworks@gmail.com/ [2] https://lore.kernel.org/netdev/20260401041929.12392-1-CFSworks@gmail.com/ [3] https://lore.kernel.org/netdev/ad-LAB08-_rpmMzK@shell.armlinux.org.uk/ --- .../net/ethernet/stmicro/stmmac/stmmac_main.c | 19 ++++++++++++------- 1 file changed, 12 insertions(+), 7 deletions(-) diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c index ca68248dbc78..3591755ea30b 100644 --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c @@ -5549,9 +5549,12 @@ static int stmmac_rx_zc(struct stmmac_priv *priv, int limit, u32 queue) break; /* Prefetch the next RX descriptor */ - rx_q->cur_rx = STMMAC_NEXT_ENTRY(rx_q->cur_rx, - priv->dma_conf.dma_rx_size); - next_entry = rx_q->cur_rx; + next_entry = STMMAC_NEXT_ENTRY(rx_q->cur_rx, + priv->dma_conf.dma_rx_size); + if (unlikely(next_entry == rx_q->dirty_rx)) + break; + + rx_q->cur_rx = next_entry; np = stmmac_get_rx_desc(priv, rx_q, next_entry); @@ -5686,7 +5689,6 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue) dma_dir = page_pool_get_dma_dir(rx_q->page_pool); bufsz = DIV_ROUND_UP(priv->dma_conf.dma_buf_sz, PAGE_SIZE) * PAGE_SIZE; - limit = min(priv->dma_conf.dma_rx_size - 1, (unsigned int)limit); if (netif_msg_rx_status(priv)) { void *rx_head = stmmac_get_rx_desc(priv, rx_q, 0); @@ -5733,9 +5735,12 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue) if (unlikely(status & dma_own)) break; - rx_q->cur_rx = STMMAC_NEXT_ENTRY(rx_q->cur_rx, - priv->dma_conf.dma_rx_size); - next_entry = rx_q->cur_rx; + next_entry = STMMAC_NEXT_ENTRY(rx_q->cur_rx, + priv->dma_conf.dma_rx_size); + if (unlikely(next_entry == rx_q->dirty_rx)) + break; + + rx_q->cur_rx = next_entry; np = stmmac_get_rx_desc(priv, rx_q, next_entry); -- 2.52.0