CVE-2026-98361 in Linux
Summary
by MITRE • 10/06/2026
In the Linux kernel, the following vulnerability has been resolved:
RDMA/rxe: Restore HMM_PFN_WRITE check in ODP write paths
Commit 0b261d7c1cd3 ("RDMA/rxe: Break endless pagefault loop for RO pages") dropped the access permission test from rxe_check_pagefault() and left only HMM_PFN_VALID. A page faulted in read-only, for example a page-cache folio behind a PROT_READ file mapping, then satisfies the check and ODP write operations (RDMA WRITE, RDMA READ response, SEND payload, atomics) modify it through kmap without ever breaking CoW.
An unprivileged user can register an ODP MR over such a mapping and have incoming RDMA traffic overwrite the page cache of a file it only holds O_RDONLY, including /etc/passwd or setuid binaries. This is the same primitive class as Dirty COW and CVE-2022-2590.
mlx5 has the missing invariant: its ODP path sets the device write bit only for pfns that carry HMM_PFN_WRITE. Restore it in rxe by requiring HMM_PFN_WRITE in rxe_check_pagefault() for every operation except RXE_PAGEFAULT_RDONLY. A write to a non-writable VMA now fails the one fault attempt with -EPERM from hmm_vma_fault() instead of re-faulting forever. For a writable VMA the fault breaks CoW and the write lands in the private page.
Keep pmem flushes on the read-only check. arch_wb_cache_pmem() never modifies memory, and the FLUSH access bits do not make the umem writable, so classifying flushes as writes would make every flush against a flush-only MR fail.
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Analysis
by VulDB Data Team • 10/06/2026
The Linux kernel's RDMA/rxe subsystem contained a critical logic error in its On-Demand Paging (ODP) write path that allowed unprivileged users to bypass memory protection mechanisms and perform unauthorized modifications to read-only file mappings. This vulnerability stems from commit 0b261d7c1cd3, which was originally intended to break an endless page fault loop for read-only pages by removing the access permission test from rxe_check_pagefault(). However, this change inadvertently left only the HMM_PFN_VALID check in place, creating a dangerous loophole. When a user registers an ODP Memory Region (MR) over a file mapping that is protected with PROT_READ but not PROT_WRITE, such as a page-cache folio behind a read-only open file descriptor, the system incorrectly treats it as writable for RDMA operations. Consequently, incoming RDMA traffic can overwrite these pages through kmap without triggering Copy-on-Write mechanisms or respecting the underlying virtual memory area permissions.
The operational impact of this flaw is severe and aligns with the Dirty COW vulnerability class, specifically mirroring the primitive used in CVE-2022-2590. An unprivileged attacker can exploit this to modify critical system files such as /etc/passwd or setuid binaries by registering an ODP MR over a read-only mapping of these files and then initiating RDMA WRITE operations, RDMA READ responses with write-back semantics, SEND payloads, or atomic operations that target the mapped memory. Because the kernel fails to enforce the HMM_PFN_WRITE flag during page fault handling for these specific RDMA paths, the protection provided by standard Unix file permissions is effectively bypassed at the hardware acceleration layer. This allows local privilege escalation and integrity compromise of system binaries without requiring elevated privileges initially.
The root cause lies in the absence of a check for the HMM_PFN_WRITE flag within rxe_check_pagefault() when processing write-oriented RDMA operations. In contrast, other drivers like mlx5 correctly maintain this invariant by setting device write bits only for page frame numbers that carry the HMM_PFN_WRITE attribute. The fix restores this critical security boundary by requiring HMM_PPFN_WRITE in rxe_check_pagefault() for all operations except those explicitly marked as RXE_PAGEFAULT_RDONLY. This ensures that writes to non-writable Virtual Memory Areas fail early with an -EPERM error from hmm_vma_fault(), preventing the infinite page fault loop and stopping unauthorized modifications before they occur. For writable VMAs, the system correctly breaks Copy-on-Write, ensuring that writes land in private pages rather than modifying shared read-only file cache data.
Mitigation strategies involve applying the kernel patch that restores the HMM_PFN_WRITE check to rxe_check_pagefault(). Administrators should ensure their systems are updated with a version of the Linux kernel containing this fix. From an industry standard perspective, this vulnerability is classified under CWE-269: Improper Privilege Management and CWE-787: Out-of-bounds Write due to incorrect boundary checks on memory access permissions. In terms of ATT&CK mapping, it relates to T1055: Process Injection or more accurately T1499: Endpoint Denial of Service if exploited for disruption, but primarily represents a Local Privilege Escalation vector (T1068) by allowing modification of security-critical files. Security teams should monitor for attempts to register ODP MRs over read-only file descriptors and verify that kernel updates are applied promptly to close this gap in RDMA memory protection logic.