CVE-2026-46110: net: stmmac: Prevent NULL deref when RX memory exhausted

In the Linux kernel, the following vulnerability has been resolved:

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, stmmacrx() bookmarks its position in the ring with the currx index. The main receive loop in that function checks for rxdescs[currx].own=0, gives the corresponding buffer to the network stack (NULLing the pointer), and increments currx modulo the ring size. After the loop exits, stmmacrxrefill(), which bookmarks its position with dirtyrx, 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 stmmacrx() only checks OWN, it confuses full/dirty. In the past (see 'Fixes:'), there was a bug where the loop could cycle currx 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 dmarxsize - 1, but this is only a partial fix: if the previous stmmacrxrefill() 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 stmmacrxrefill() is memory-starved long enough for currx to catch up to dirtyrx.

Fix this by explicitly checking, before advancing currx, if the next entry is dirty; exit the loop if so. This prevents processing of the final, used descriptor until stmmacrxrefill() succeeds, but fully prevents the currx == dirtyrx ambiguity as the previous bugfix intended: so remove the clamp as well. Since stmmacrxzc() is a copy-paste-and-tweak of stmmacrx() and the code structure is identical, any fix to stmmacrx() will also need a corresponding fix for stmmacrxzc(). Therefore, apply the same check there.

In stmmacrx() (not stmmacrxzc()), 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 stmmacrxrefill() 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.