CVE-2026-72466 in Linux
Summary
by MITRE • 08/15/2026
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()).
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability described affects the Linux kernel's rpcrdma implementation within the xprtrdma subsystem, specifically targeting the handling of backchannel calls during RPC transport operations. This flaw resides in how the system decodes incoming reply frames to determine if they represent backchannel communications. The issue manifests through two distinct but interconnected problems that together create a persistent resource leak and potential denial-of-service condition.
The primary technical issue stems from an improper use of the xdr_inline_decode() function when processing reply headers. When rpcrdma_is_bcall() attempts to decode a reply, it makes an initial call requesting zero bytes from the XDR stream via xdr_inline_decode(xdr, 0). This operation returns the current pointer without validating whether sufficient data exists in the buffer, allowing subsequent operations to read up to 20 bytes beyond the actual wire payload. These five consecutive __be32 reads traverse into stale memory regions of previously received data, causing the system to incorrectly classify malformed or short replies as backchannel calls. This misclassification leads to improper resource management where receive buffers become orphaned and unavailable for reuse.
The secondary vulnerability occurs in the control flow following the peek operation. After processing the initial bytes, when dealing with short headers, the function returns true without invoking the proper backchannel processing routine rpcrdma_bc_receive_call(). According to the established contract within rpcrdma_reply_handler(), a true return value signifies that ownership of the reply structure has been transferred to the backchannel path. However, since rpcrdma_bc_receive_call() was never executed, no entity takes responsibility for managing the reply structure. This causes the reply object, which maintains a DMA-mapped receive buffer, to remain in the rb_all_reps list indefinitely until transport teardown occurs, effectively orphaning the buffer and creating a resource leak.
The operational impact of this vulnerability extends beyond simple memory consumption issues. The persistent leak of receive buffers results in gradual depletion of the Receive queue capacity, ultimately leading to a denial-of-service condition where legitimate network traffic cannot be properly processed due to exhausted resources. Network peers begin experiencing RNR NAKs when their send operations encounter unavailable receive slots, disrupting normal communication patterns and potentially causing cascading failures in distributed systems relying on RDMA transport mechanisms.
The fix addresses both issues through careful boundary checking and proper control flow management. First, the zero-length peek operation now includes validation against xdr->end to ensure that subsequent reads only occur when sufficient data exists in the buffer. This byte-precise comparison accounts for non-4-aligned memory boundaries that can cause word count rounding issues during stream processing. Second, the short-header code path now returns false instead of true, ensuring that replies follow the standard cleanup procedure through out_norqst chain, which properly calls rpcrdma_rep_put() and rpcrdma_post_recvs(). This maintains proper resource accounting and prevents the orphaning of DMA-mapped receive buffers.
This vulnerability aligns with CWE-129 Indirect Access to Resource Through Symbolic Name and CWE-476 NULL Pointer Dereference, while its exploitation patterns correspond to ATT&CK techniques involving resource exhaustion and denial-of-service attacks. The flaw demonstrates the importance of proper input validation in kernel network processing code and highlights how seemingly minor API usage errors can cascade into significant system stability issues.
The resolution ensures that all receive buffer management operations maintain proper state transitions and that resource leak prevention mechanisms function correctly even under malformed packet conditions. This fix strengthens the overall robustness of RDMA-based RPC implementations by ensuring proper lifecycle management of DMA-mapped resources and maintaining predictable resource consumption patterns.