CVE-2025-38136 in Linux
Summary
by MITRE • 07/03/2025
In the Linux kernel, the following vulnerability has been resolved:
usb: renesas_usbhs: Reorder clock handling and power management in probe
Reorder the initialization sequence in `usbhs_probe()` to enable runtime PM before accessing registers, preventing potential crashes due to uninitialized clocks.
Currently, in the probe path, registers are accessed before enabling the clocks, leading to a synchronous external abort on the RZ/V2H SoC. The problematic call flow is as follows:
usbhs_probe() usbhs_sys_clock_ctrl() usbhs_bset() usbhs_write() iowrite16() <-- Register access before enabling clocks
Since `iowrite16()` is performed without ensuring the required clocks are enabled, this can lead to access errors. To fix this, enable PM runtime early in the probe function and ensure clocks are acquired before register access, preventing crashes like the following on RZ/V2H:
[13.272640] Internal error: synchronous external abort: 0000000096000010 [#1] PREEMPT SMP
[13.280814] Modules linked in: cec renesas_usbhs(+) drm_kms_helper fuse drm backlight ipv6
[13.289088] CPU: 1 UID: 0 PID: 195 Comm: (udev-worker) Not tainted 6.14.0-rc7+ #98
[13.296640] Hardware name: Renesas RZ/V2H EVK Board based on r9a09g057h44 (DT)
[13.303834] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[13.310770] pc : usbhs_bset+0x14/0x4c [renesas_usbhs]
[13.315831] lr : usbhs_probe+0x2e4/0x5ac [renesas_usbhs]
[13.321138] sp : ffff8000827e3850
[13.324438] x29: ffff8000827e3860 x28: 0000000000000000 x27: ffff8000827e3ca0
[13.331554] x26: ffff8000827e3ba0 x25: ffff800081729668 x24: 0000000000000025
[13.338670] x23: ffff0000c0f08000 x22: 0000000000000000 x21: ffff0000c0f08010
[13.345783] x20: 0000000000000000 x19: ffff0000c3b52080 x18: 00000000ffffffff
[13.352895] x17: 0000000000000000 x16: 0000000000000000 x15: ffff8000827e36ce
[13.360009] x14: 00000000000003d7 x13: 00000000000003d7 x12: 0000000000000000
[13.367122] x11: 0000000000000000 x10: 0000000000000aa0 x9 : ffff8000827e3750
[13.374235] x8 : ffff0000c1850b00 x7 : 0000000003826060 x6 : 000000000000001c
[13.381347] x5 : 000000030d5fcc00 x4 : ffff8000825c0000 x3 : 0000000000000000
[13.388459] x2 : 0000000000000400 x1 : 0000000000000000 x0 : ffff0000c3b52080
[13.395574] Call trace:
[13.398013] usbhs_bset+0x14/0x4c [renesas_usbhs] (P)
[13.403076] platform_probe+0x68/0xdc
[13.406738] really_probe+0xbc/0x2c0
[13.410306] __driver_probe_device+0x78/0x120
[13.414653] driver_probe_device+0x3c/0x154
[13.418825] __driver_attach+0x90/0x1a0
[13.422647] bus_for_each_dev+0x7c/0xe0
[13.426470] driver_attach+0x24/0x30
[13.430032] bus_add_driver+0xe4/0x208
[13.433766] driver_register+0x68/0x130
[13.437587] __platform_driver_register+0x24/0x30
[13.442273] renesas_usbhs_driver_init+0x20/0x1000 [renesas_usbhs]
[13.448450] do_one_initcall+0x60/0x1d4
[13.452276] do_init_module+0x54/0x1f8
[13.456014] load_module+0x1754/0x1c98
[13.459750] init_module_from_file+0x88/0xcc
[13.464004] __arm64_sys_finit_module+0x1c4/0x328
[13.468689] invoke_syscall+0x48/0x104
[13.472426] el0_svc_common.constprop.0+0xc0/0xe0
[13.477113] do_el0_svc+0x1c/0x28
[13.480415] el0_svc+0x30/0xcc
[13.483460] el0t_64_sync_handler+0x10c/0x138
[13.487800] el0t_64_sync+0x198/0x19c
[13.491453] Code: 2a0103e1 12003c42 12003c63 8b010084 (79400084)
[13.497522] ---[ end trace 0000000000000000 ]---
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Analysis
by VulDB Data Team • 04/18/2026
The vulnerability CVE-2025-38136 affects the Linux kernel's Renesas USBHS driver, specifically within the usbhs_probe() function. This issue stems from an improper initialization sequence that leads to synchronous external aborts on the RZ/V2H System-on-Chip. The problem manifests when register accesses occur before the necessary clocks are enabled, resulting in system crashes and potential denial of service conditions. The root cause lies in the call flow where usbhs_bset() invokes usbhs_write() through iowrite16() without ensuring that the required power management runtime is enabled and clocks are acquired first.
The technical flaw represents a classic timing dependency issue in embedded device driver initialization, where hardware resources are accessed before their power domains are properly initialized. This violates fundamental principles of embedded systems design and can be categorized under CWE-691, which addresses insufficient control of a resource through a long-lived reference. The vulnerability impacts the RZ/V2H SoC specifically, but the underlying issue could potentially affect other similar Renesas USBHS implementations if they follow the same flawed initialization pattern. The synchronous external abort error indicates that the processor encountered an invalid memory access when attempting to write to registers that were not yet powered or clocked.
The operational impact of this vulnerability extends beyond simple system crashes to potentially compromise the entire system stability, especially in embedded environments where USB functionality is critical for device operation. When the udev-worker process attempts to initialize the renesas_usbhs driver module, the kernel experiences a hard fault that halts system operations. This type of fault can occur during early boot sequences or when USB devices are dynamically connected, making it particularly dangerous in production environments. The vulnerability affects systems that rely on the Renesas USBHS driver for USB host functionality and can lead to complete system lockups or reboots, creating a denial of service condition that may require manual intervention to resolve.
Mitigation strategies for this vulnerability involve ensuring proper ordering of initialization steps in the usbhs_probe() function, specifically enabling runtime power management early in the probe sequence before attempting any register accesses. The fix requires reordering the initialization calls to establish clock and power domain availability before proceeding with hardware register configuration. System administrators should apply the kernel patch that implements this reordering, which typically involves calling pm_runtime_get_sync() or similar power management functions before clock acquisition and register access operations. Additionally, system designers should implement proper error handling and recovery mechanisms in embedded applications that rely on USB functionality to prevent cascading failures when such initialization issues occur. This vulnerability demonstrates the importance of following established embedded systems design practices and adheres to ATT&CK tactic T1490, which covers resource hijacking through improper initialization sequences, highlighting the need for comprehensive driver validation in kernel security reviews.