CVE-2026-98431 in Linux
Summary
by MITRE • 10/15/2026
In the Linux kernel, the following vulnerability has been resolved:
RDMA/uverbs: Fix mmap_lock/disassociation_lock circular dependency
Commit 51976c6cd786 ("RDMA/core: Provide rdma_user_mmap_disassociate() to disassociate mmap pages") introduced disassociation_lock to protect new mmap registrations against uverbs_user_mmap_disassociate(), but created an ABBA deadlock:
Thread A (mmap / fork): mmap_lock -> disassociation_lock
Thread B (disassociate): disassociation_lock -> mmap_lock
Fix by removing disassociation_lock entirely and using the pre-existing hw_destroy_rwsem instead. hw_destroy_rwsem already provides the same protection: rdma_umap_open() and ib_uverbs_mmap() both use down_read_trylock() before registering a new VMA, so holding hw_destroy_rwsem in uverbs_user_mmap_disassociate() is sufficient to block new registrations. trylock is used in both mmap paths (not blocking down_read) because mmap_lock is already held on entry, and uverbs_user_mmap_disassociate() acquires mmap_lock internally — a blocking read would recreate the same deadlock.
The only caller that was not taking hw_destroy_rwsem for write was rdma_user_mmap_disassociate(). Fix it to take the rwsem per-ufile while iterating under lists_mutex. This is safe because ib_uverbs_close() releases hw_destroy_rwsem entirely before acquiring lists_mutex, so the two locks are never held simultaneously.
lockdep warning:
[ 776.654252] ======================================================
[ 776.655214] WARNING: possible circular locking dependency detected
[ 776.656167] 6.18.0for-upstream_debug_94e244d9ccab #1 Not tainted
[ 776.657114] ------------------------------------------------------
[ 776.658087] devlink/14824 is trying to acquire lock:
[ 776.658879] ffff88811170c800 (&mm->mmap_lock){++++}-{4:4}, at: uverbs_user_mmap_disassociate+0x168/0x780 [ib_uverbs]
[ 776.660479]
[ 776.660479] but task is already holding lock:
[ 776.661460] ffff888142d92b08 (&file->disassociation_lock){+.+.}-{4:4}, at: uverbs_user_mmap_disassociate+0x39/0x780 [ib_uverbs]
[ 776.663177]
[ 776.663177] which lock already depends on the new lock.
[ 776.663177]
[ 776.664525]
[ 776.664525] the existing dependency chain (in reverse order) is:
[ 776.665724]
[ 776.665724] -> #2 (&file->disassociation_lock){+.+.}-{4:4}:
[ 776.666887] __mutex_lock+0x16d/0x2330
[ 776.667633] rdma_umap_open+0x129/0x280 [ib_uverbs]
[ 776.668489] dup_mmap+0xa40/0x1790
[ 776.669170] copy_process+0x5dd2/0x6170
[ 776.669933] kernel_clone+0xb6/0x610
[ 776.670636] __do_sys_clone+0xb5/0xf0
[ 776.671354] do_syscall_64+0x70/0x12e0
[ 776.672083] entry_SYSCALL_64_after_hwframe+0x4b/0x53
[ 776.672940]
[ 776.672940] -> #1 (&mm->mmap_lock/1){+.+.}-{4:4}:
[ 776.673985] down_write_nested+0x90/0x1e0
[ 776.674751] dup_mmap+0x201/0x1790
[ 776.675448] copy_process+0x5dd2/0x6170
[ 776.676180] kernel_clone+0xb6/0x610
[ 776.676904] __do_sys_clone+0xb5/0xf0
[ 776.677615] do_syscall_64+0x70/0x12e0
[ 776.678351] entry_SYSCALL_64_after_hwframe+0x4b/0x53
[ 776.679239]
[ 776.679239] -> #0 (&mm->mmap_lock){++++}-{4:4}:
[ 776.680253] __lock_acquire+0x18c6/0x2ec0
[ 776.681018] lock_acquire+0x10e/0x2e0
[ 776.681742] down_read+0x95/0x430
[ 776.682395] uverbs_user_mmap_disassociate+0x168/0x780 [ib_uverbs]
[ 776.683436] uverbs_destroy_ufile_hw+0x1ae/0x270 [ib_uverbs]
[ 776.684416] ib_uverbs_remove_one+0x22b/0x420 [ib_uverbs]
[ 776.685371] remove_client_context+0xa6/0xf0 [ib_core]
[ 776.686342] disable_device+0x12b/0x240 [ib_core]
[ 776.687249] __ib_unregister_device+0x269/0x460 [ib_core]
[ 776.688233] ib_unregister_device+0x21/0x30 [ib_core]
[ 776.689140] mlx5r_remove+0xd0/0x170 [mlx5_ib]
[ 776.689999] device_release_driver_internal+0x3b2/0x5
---truncated---
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Analysis
by VulDB Data Team • 10/15/2026
The Linux kernel's RDMA user-space interface subsystem contained a critical concurrency flaw resulting in an ABBA deadlock, identified by commit 51976c6cd786. This vulnerability arose from the introduction of disassociation_lock to protect new memory map registrations against concurrent unregistration operations. The architectural design created two distinct execution paths with opposing lock acquisition orders. In one path, triggered during process forking or initial mapping via dup_mmap and rdma_umap_open, the kernel acquires mmap_lock followed by disassociation_lock. Conversely, in the other path initiated by device removal or context destruction through uverbs_user_mmap_disassociate, the system attempts to acquire disassociation_lock first and then mmap_lock. This inversion of lock ordering creates a classic circular dependency where Thread A holds the first lock and waits for the second, while Thread B holds the second lock and waits for the first, leading to an indefinite block that effectively causes a denial of service within the kernel context.
The operational impact of this vulnerability is significant as it can be triggered by standard system calls such as fork or clone, which duplicate memory maps, or during hardware device unregistration processes involving InfiniBand devices like those from Mellanox Technologies. When lockdep detects this circular dependency, it halts further locking operations to prevent corruption, effectively freezing the affected kernel thread and potentially impacting system stability if critical resources are held by the deadlocked process. The vulnerability falls under CWE-833 Lock Inversion, a category of concurrency errors where multiple locks are acquired in an inconsistent order across different execution paths. From a threat modeling perspective using MITRE ATT&CK techniques, this aligns with T1499 Endpoint Denial of Service, as the deadlock renders specific kernel subsystems unresponsive to legitimate requests until the system is rebooted or the offending process is forcefully terminated by external intervention.
The resolution involves removing the redundant disassociation_lock entirely and relying on the pre-existing hw_destroy_rwsem semaphore for synchronization. This rwsem provides sufficient protection because both rdma_umap_open and ib_uverbs_mmap utilize down_read_trylock before registering a new Virtual Memory Area, ensuring that no new mappings can be established while an active disassociation is in progress. The fix carefully maintains non-blocking behavior by using trylock variants in the mmap paths since mmap_lock is already held upon entry to these functions. Acquiring hw_destroy_rwsem with a blocking read lock inside uverbs_user_mmap_disassociate would recreate the deadlock because it also requires holding mmap_lock internally. Therefore, the solution leverages the existing hierarchical locking structure where ib_uverbs_close releases hw_destroy_rwsem before acquiring lists_mutex, ensuring that no two locks are held simultaneously in conflicting orders during device teardown operations.
Mitigation strategies for this vulnerability primarily involve applying the upstream kernel patch associated with commit 51976c6cd786 to update the RDMA uverbs subsystem codebase. System administrators managing environments with high rates of process creation or frequent hardware device hot-plugging and unregistration should prioritize these updates to prevent potential service interruptions. For organizations unable to immediately apply patches, monitoring for lockdep warnings in kernel logs can serve as an early detection mechanism for similar concurrency issues within the RDMA stack. Long-term architectural improvements should focus on enforcing strict global lock ordering policies across all kernel subsystems to eliminate the possibility of circular dependencies arising from disparate development teams implementing protection mechanisms independently without full awareness of existing locking hierarchies.