Out-of-bounds Read Affecting kernel6.18-tools-devel package, versions <1:6.18.41-94.142.amzn2023

Note: Versions mentioned in the description apply only to the upstream kernel6.18-tools-devel package and not the kernel6.18-tools-devel package as distributed by Amazon-Linux. See How to fix? for Amazon-Linux:2023 relevant fixed versions and status.

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

xprtrdma: Fix bcall rep leak and unbounded peek

rpcrdma_is_bcall() decodes a reply's first words to decide whether the frame is a backchannel call. Two issues in that decode path let a short or malformed reply leak the receive buffer and drain the Receive queue.

First, the speculative peek

p = xdr_inline_decode(xdr, 0);
/* five p++ reads follow */

asks xdr_inline_decode() for zero bytes, which returns xdr->p without consulting xdr->end. The five subsequent __be32 reads can then walk up to 20 bytes past the wire payload into stale regbuf contents and misclassify the reply as a backchannel call.

Second, after the post-peek

p = xdr_inline_decode(xdr, 3 * sizeof(*p));
if (unlikely(!p))
        return true;

the short-header arm returns true without calling rpcrdma_bc_receive_call(). The contract with the caller is that a true return transfers ownership of rep to the backchannel path:

rpcrdma_reply_handler()
  if (rpcrdma_is_bcall(r_xprt, rep))
          return;        /* bare return, skips out_post */
  ...
out_post:
  rpcrdma_post_recvs(r_xprt, credits + ...);

Because rpcrdma_bc_receive_call() never ran, no one took rep, but rpcrdma_reply_handler still bare-returns past rpcrdma_rep_put() and rpcrdma_post_recvs(). The rep, with its persistently DMA-mapped receive buffer, is orphaned on rb_all_reps and freed only at transport teardown. This completion reposts nothing, so its slot is reclaimed only when a later forward-channel reply reaches out_post and rpcrdma_post_recvs() allocates a fresh rep to backfill; absent that traffic the Receive queue drains and the peer's Sends draw RNR NAKs.

Fix by consulting xdr->end after the zero-length peek so the five __be32 reads cannot run unless 20 bytes of wire payload remain. A byte-precise comparison against xdr->end is required because a non-4-aligned receive rounds the stream's word count up past the true payload. Also return false from the short-header arm so the reply falls through the normal out_norqst cleanup chain (rpcrdma_rep_put() plus rpcrdma_post_recvs()).

• http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-72466
• https://git.kernel.org/stable/c/07aa506436be7634e381e1e1f6d0efa9efc81ecc
• https://git.kernel.org/stable/c/0cee8f9c3b14bd6dee9c4310090a7f45b89b834f
• https://git.kernel.org/stable/c/118a16a18c59f7ad8084b2d13988839b669fca10
• https://git.kernel.org/stable/c/7afc2f8d2fd9394724df9eaf22ce7a71029a5fba
• https://git.kernel.org/stable/c/88b5346284a184a6b7d019912232a571d672d3e3
• https://git.kernel.org/stable/c/c7653d5cebc8492c77ec0415b5e9c0fb3e644bc6
• https://git.kernel.org/stable/c/d7a2870dde3bb09d51d6b9c877642996ad6b92dd