[PATCH net v5] net: stmmac: Prevent NULL deref when RX memory exhausted

[Sam Edwards <cfsworks@gmail.com>]

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.

Fixes: b6cb4541853c7 ("net: stmmac: avoid rx queue overrun")
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=221010
Cc: stable@vger.kernel.org
Signed-off-by: Sam Edwards <CFSworks@gmail.com>
---

Hi list,

This is a single patch broken out of [1]. The second patch in that series,
which proactively refills the RX ring buffer when memory is low, still has some
unresolved feedback: it should use a timer to avoid nuisance polling while the
system is suffering OOM.

Further discussion makes me wonder whether that second patch should even be
threshold-triggered at all, or if it should be a handler for the RBU
("Receive Buffer Unavailable") interrupt instead.

So, while that patch is back at the drawing board, I am submitting this one
(which is higher-priority as it resolves a *panic*) separately.

Regards,
Sam

[1] https://lore.kernel.org/all/20260401041929.12392-1-CFSworks@gmail.com/

---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 13d3cac056be..fc11f75f7dc0 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -5609,7 +5609,8 @@ 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);
+	limit = min(priv->dma_conf.dma_rx_size - stmmac_rx_dirty(priv, queue) - 1,
+		    (unsigned int)limit);
 
 	if (netif_msg_rx_status(priv)) {
 		void *rx_head;
-- 
2.52.0