CVE-2026-31649
CriticalCVSS 9.8Exploitation Probability (EPSS)
Low risk33th percentile - higher than 33% of all known CVEs
Summary
In the Linux kernel, the stmmac driver has an integer underflow vulnerability in chain mode. The jumbo_frm() function computes len = nopaged_len - bmax, which wraps when the linear portion is small but total packet size is large, causing excessive iterations and mapping arbitrary kernel memory to DMA. This leads to kernel memory disclosure and potential memory corruption.
Risk Assessment
The organization risks exposure of sensitive kernel memory and potential system corruption via DMA hardware, which could compromise data integrity and cause system crashes.
Recommendation
Immediately update the Linux kernel to a version containing the fix that introduces a buf_len variable clamped to min(nopaged_len, bmax), preventing the integer underflow.
Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: net: stmmac: fix integer underflow in chain mode The jumbo_frm() chain-mode implementation unconditionally computes len = nopaged_len - bmax; where nopaged_len = skb_headlen(skb) (linear bytes only) and bmax is BUF_SIZE_8KiB or BUF_SIZE_2KiB. However, the caller stmmac_xmit() decides to invoke jumbo_frm() based on skb->len (total length including page fragments): is_jumbo = stmmac_is_jumbo_frm(priv, skb->len, enh_desc); When a packet has a small linear portion (nopaged_len <= bmax) but a large total length due to page fragments (skb->len > bmax), the subtraction wraps as an unsigned integer, producing a huge len value (~0xFFFFxxxx). This causes the while (len != 0) loop to execute hundreds of thousands of iterations, passing skb->data + bmax * i pointers far beyond the skb buffer to dma_map_single(). On IOMMU-less SoCs (the typical deployment for stmmac), this maps arbitrary kernel memory to the DMA engine, constituting a kernel memory disclosure and potential memory corruption from hardware. Fix this by introducing a buf_len local variable clamped to min(nopaged_len, bmax). Computing len = nopaged_len - buf_len is then always safe: it is zero when the linear portion fits within a single descriptor, causing the while (len != 0) loop to be skipped naturally, and the fragment loop in stmmac_xmit() handles page fragments afterward.