CVE-2026-31649net: stmmac: fix integer underflow in chain mode

Official description

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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/513e06735f5be575b409d195822195348b164e48
• https://git.kernel.org/stable/c/275bdf762e82082f064e60a92448fa2ac43cf95b
• https://git.kernel.org/stable/c/a2b68a9a476b9544ff31f1fbcd5d80867a8a5e2f
• https://git.kernel.org/stable/c/b7b8012193fd98236d7ae05d4b553f010a77b2ef
• https://git.kernel.org/stable/c/2c91b39912278d0878f9ba60ba04d2518b18a08d
• https://git.kernel.org/stable/c/6fca757c20396dc2e604dcc61922264e9e3dc803
• https://git.kernel.org/stable/c/10d12b9240ebf96c785f0e2e4228318cd5f3a3eb
• https://git.kernel.org/stable/c/51f4e090b9f87b40c21b6daadb5c06e6c0a07b67
• https://cert-portal.siemens.com/productcert/html/ssa-082556.html
• https://cert-portal.siemens.com/productcert/html/ssa-019113.html
• https://cert-portal.siemens.com/productcert/html/ssa-457936.html