CVE-2026-89784 in Linux
Summary
by MITRE • 09/16/2026
In the Linux kernel, the following vulnerability has been resolved:
SUNRPC: check rpc_sockaddr2uaddr() return value in rpcb_register_inet4/6
rpcb_register_inet4() and rpcb_register_inet6() store the result of rpc_sockaddr2uaddr() into map->r_addr without checking it for NULL. rpc_sockaddr2uaddr() returns NULL when its final kstrdup() fails, and the unchecked NULL is then carried into the synchronous RPCBPROC_SET encode path: rpcb_register_call() -> rpc_call_sync() -> rpcb_enc_getaddr() -> encode_rpcb_string(), whose first statement is strlen(string), dereferencing NULL and oopsing the kernel.
The crash reproduces under failslab on v6.12; with KASAN the NULL dereference surfaces as a fault on the shadow of address zero:
Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000 [#1] PREEMPT SMP KASAN
RIP: 0010:strlen (lib/string.c:409) Call Trace: encode_rpcb_string (net/sunrpc/rpcb_clnt.c:890) rpcb_enc_getaddr (net/sunrpc/rpcb_clnt.c:910) rpcauth_wrap_req_encode (net/sunrpc/auth.c:745) call_encode (net/sunrpc/clnt.c:1966) __rpc_execute (net/sunrpc/sched.c:952) rpc_run_task (net/sunrpc/clnt.c:1243) rpc_call_sync (net/sunrpc/clnt.c:1272) rpcb_v4_register (net/sunrpc/rpcb_clnt.c:500) svc_generic_rpcbind_set nfsd_rpcbind_set svc_register svc_setup_socket svc_addsock write_ports nfsctl_transaction_write vfs_write
The crash is reachable when an in-kernel RPC service (nfsd, lockd, nfs-callback) registers with the local rpcbind under enough memory pressure for the small GFP_KERNEL kstrdup() in rpc_sockaddr2uaddr() to fail. The asynchronous getport path already handles this exact failure mode by returning -ENOMEM; only the two register helpers omit the check.
Mirror that handling: bail out with -ENOMEM when rpc_sockaddr2uaddr() returns NULL, before the address is fed into the encoder.
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Analysis
by VulDB Data Team • 09/16/2026
The Linux kernel contains a critical null pointer dereference vulnerability within the SUNRPC subsystem, specifically affecting the RPCB registration functions for IPv4 and IPv6 addresses. This flaw resides in the rpcb_register_inet4() and rpcb_register_inet6() routines, which are responsible for registering local network services with the system's rpcbind daemon. The core technical issue stems from an unchecked return value when invoking the helper function rpc_sockaddr2uaddr(). Under normal operating conditions, this function converts a socket address structure into a universal address representation by allocating memory via kstrdup. However, if the kernel is under significant memory pressure or subjected to allocation failure simulations such as failslab tests, kstrdup may return NULL instead of a valid pointer. The vulnerable code paths blindly store this potentially null result directly into the map->r_addr field without performing any validity checks before proceeding with subsequent operations.
This unchecked null pointer propagates through the synchronous RPCBPROC_SET encode path, triggering a chain of function calls that ultimately leads to a kernel panic or general protection fault. Specifically, after storing the address, the code invokes rpcb_register_call(), which executes an synchronous RPC call via rpc_call_sync(). This triggers the encoder routine rpcb_enc_getaddr(), followed by encode_rpcb_string(). The first operation within encode_rpcb_string() is to calculate the length of the string using strlen on the map->r_addr pointer. Because this pointer holds a NULL value due to the earlier allocation failure, dereferencing it causes an immediate crash. In environments with Kernel Address Sanitizer enabled, this manifests as a fault on the shadow memory address zero, while standard execution results in a general protection fault indicating access to a non-canonical or null address within lib/string.c during strlen execution.
The operational impact of this vulnerability is severe, resulting in local denial of service through kernel oopsing and system instability. The attack vector requires an attacker with the ability to trigger RPCB registration operations while simultaneously inducing memory pressure on the host system. This scenario can occur when in-kernel RPC services such as nfsd (Network File System server), lockd (NLM daemon), or NFS callbacks attempt to register their endpoints with the local rpcbind service. If the small GFP_KERNEL allocation required for string duplication fails due to resource exhaustion, the kernel will crash rather than gracefully handling the error condition. This represents a significant reliability issue in production environments where memory fragmentation or targeted denial-of-service attacks could exploit this path to destabilize critical file sharing and locking infrastructure services running on Linux systems.
From a classification perspective, this vulnerability aligns with CWE-476, which denotes NULL Pointer Dereference, as the root cause is the failure to verify that a pointer returned from an allocation function is non-null before use. It also relates to CWE-252, Unchecked Return Value, because the code ignores the potential error indication provided by rpc_sockaddr2uaddr(). In terms of adversarial tactics, this flaw could be leveraged within ATT&CK technique T1498, Network Denial of Service, specifically through resource exhaustion leading to service disruption. The vulnerability highlights a discrepancy in defensive coding practices within the same subsystem, as the asynchronous getport path already correctly handles allocation failures by returning -ENOMEM, whereas the synchronous registration helpers omitted this essential safety check.
To mitigate this risk, developers must implement explicit null checks immediately following any call to rpc_sockaddr2uaddr() within the register helper functions. If the function returns NULL, indicating an allocation failure, the code should abort the current operation and return -ENOMEM to signal the error condition up the stack rather than proceeding with invalid data. This mirrors the existing robust handling found in other parts of the SUNRPC subsystem and ensures that memory pressure does not lead to kernel instability. System administrators can temporarily reduce exposure by monitoring system memory usage and ensuring adequate resources are available for critical network services, although a code patch is required to fully resolve the defect. Regular auditing of error handling paths in low-level networking drivers and kernel modules is essential to prevent similar null pointer dereferences that compromise system integrity.