net: stmmac: Prevent NULL deref when RX memory exhausted

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 [email protected]

Suggested-by Russell King <[email protected]>

Signed-off-by Sam Edwards <[email protected]>

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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/[email protected]/
[2] https://lore.kernel.org/netdev/[email protected]/
[3] https://lore.kernel.org/netdev/[email protected]/

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 .../net/ethernet/stmicro/stmmac/stmmac_main.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c +0 -0

index ca68248dbc78..3591755ea30b 100644