CVE-2026-31649

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.