Linux Kernel stmmac Integer Underflow in Chain Mode
CVE-2026-31649 Published on April 24, 2026
net: stmmac: fix integer underflow in chain mode
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.
Products Associated with CVE-2026-31649
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Affected Versions
Linux:- Version 286a837217204b1ef105e3a554d0757e4fdfaac1 and below 513e06735f5be575b409d195822195348b164e48 is affected.
- Version 286a837217204b1ef105e3a554d0757e4fdfaac1 and below 275bdf762e82082f064e60a92448fa2ac43cf95b is affected.
- Version 286a837217204b1ef105e3a554d0757e4fdfaac1 and below a2b68a9a476b9544ff31f1fbcd5d80867a8a5e2f is affected.
- Version 286a837217204b1ef105e3a554d0757e4fdfaac1 and below b7b8012193fd98236d7ae05d4b553f010a77b2ef is affected.
- Version 286a837217204b1ef105e3a554d0757e4fdfaac1 and below 2c91b39912278d0878f9ba60ba04d2518b18a08d is affected.
- Version 286a837217204b1ef105e3a554d0757e4fdfaac1 and below 6fca757c20396dc2e604dcc61922264e9e3dc803 is affected.
- Version 286a837217204b1ef105e3a554d0757e4fdfaac1 and below 10d12b9240ebf96c785f0e2e4228318cd5f3a3eb is affected.
- Version 286a837217204b1ef105e3a554d0757e4fdfaac1 and below 51f4e090b9f87b40c21b6daadb5c06e6c0a07b67 is affected.
- Version 3.2 is affected.
- Before 3.2 is unaffected.
- Version 5.10.253, <= 5.10.* is unaffected.
- Version 5.15.203, <= 5.15.* is unaffected.
- Version 6.1.169, <= 6.1.* is unaffected.
- Version 6.6.135, <= 6.6.* is unaffected.
- Version 6.12.82, <= 6.12.* is unaffected.
- Version 6.18.23, <= 6.18.* is unaffected.
- Version 6.19.13, <= 6.19.* is unaffected.
- Version 7.0, <= * is unaffected.