CVE-2026-97953

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

net: stmmac: fix TX descriptor availability check for TSO traffic

stmmac_tso_xmit() estimates the number of free TX descriptors required by

a TSO skb as:

(skb->len - proto_hdr_len) / TSO_MAX_BUFF_SIZE + 1

which assumes the payload is split into TSO_MAX_BUFF_SIZE chunks. This

underestimates the descriptors actually consumed by stmmac_tso_allocator(),

since each fragment is mapped individually and so it needs at least one

descriptor regardless of its size. Moreover, one descriptor is used for

the L2/L3/L4 headers and, when the MSS changes, one more is consumed for

the MSS context descriptor.

For a highly fragmented TSO skb the check can therefore pass even when the

ring has too few free slots. stmmac_tso_allocator() then writes past the

available descriptors, overwriting descriptors still owned by the DMA

engine, corrupting the TX ring.

Add stmmac_tso_get_num_desc() to compute the exact number of descriptors

needed for the header, the linear payload and each fragment, plus the MSS

context descriptor when required, and use it in the availability check.