[PATCH net v6] 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 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 <linux@armlinux.org.uk>
Signed-off-by: Sam Edwards <CFSworks@gmail.com>
---

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