HIGH
ublk RefCount Leak
CVE-2025-71070
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:H
01Description
In the Linux kernel, the following vulnerability has been resolved: ublk: clean up user copy references on ublk server exit If a ublk server process releases a ublk char device file, any requests dispatched to the ublk server but not yet completed will retain a ref value of UBLK_REFCOUNT_INIT. Before commit e63d2228ef83 ("ublk: simplify aborting ublk request"), __ublk_fail_req() would decrement the reference count before completing the failed request. However, that commit optimized __ublk_fail_req() to call __ublk_complete_rq() directly without decrementing the request reference count. The leaked reference count incorrectly allows user copy and zero copy operations on the completed ublk request. It also triggers the WARN_ON_ONCE(refcount_read(&io->ref)) warnings in ublk_queue_reinit() and ublk_deinit_queue(). Commit c5c5eb24ed61 ("ublk: avoid ublk_io_release() called after ublk char dev is closed") already fixed the issue for ublk devices using UBLK_F_SUPPORT_ZERO_COPY or UBLK_F_AUTO_BUF_REG. However, the reference count leak also affects UBLK_F_USER_COPY, the other reference-counted data copy mode. Fix the condition in ublk_check_and_reset_active_ref() to include all reference-counted data copy modes. This ensures that any ublk requests still owned by the ublk server when it exits have their reference counts reset to 0.
02KernelScan AI Analysis
Risk summary
A reference count leak in the ublk (userspace block) driver allows use-after-free operations on completed block requests when a ublk server exits with I/O still in flight. Devices using UBLK_F_USER_COPY are affected, enabling memory corruption, limited information disclosure, and potential kernel panics.
Vulnerability analysis
Commit e63d2228ef83 optimized __ublk_fail_req() to call __ublk_complete_rq() directly without decrementing the request reference count during server abort. When a ublk server releases its character device while requests are pending, those requests retain their initial reference count (UBLK_REFCOUNT_INIT) even after completion. Because the reference count never reaches zero, ublk_io_release() is skipped and the kernel incorrectly permits further user-copy and zero-copy operations on the completed request structures. This constitutes a use-after-free primitive on block I/O requests. The bug also triggers WARN_ON_ONCE() assertions during queue reinitialization or teardown. The fix expands ublk_check_and_reset_active_ref() to cover all reference-counted copy modes, forcibly resetting active references to zero on server exit.
03Fix Versions
| Branch | Fixed in | Patch commit |
|---|---|---|
| 6.15 | 6.15 | 13456b4f1033 |
| 6.18 | 6.18.3 | daa24603d9f0 |
| mainline | 6.19 | — |