CVE-2023-52849 in Linux
Summary
by MITRE • 05/21/2024
In the Linux kernel, the following vulnerability has been resolved:
cxl/mem: Fix shutdown order
Ira reports that removing cxl_mock_mem causes a crash with the following trace:
BUG: kernel NULL pointer dereference, address: 0000000000000044 [..]
RIP: 0010:cxl_region_decode_reset+0x7f/0x180 [cxl_core]
[..]
Call Trace: cxl_region_detach+0xe8/0x210 [cxl_core]
cxl_decoder_kill_region+0x27/0x40 [cxl_core]
cxld_unregister+0x29/0x40 [cxl_core]
devres_release_all+0xb8/0x110 device_unbind_cleanup+0xe/0x70 device_release_driver_internal+0x1d2/0x210 bus_remove_device+0xd7/0x150 device_del+0x155/0x3e0 device_unregister+0x13/0x60 devm_release_action+0x4d/0x90 ? __pfx_unregister_port+0x10/0x10 [cxl_core]
delete_endpoint+0x121/0x130 [cxl_core]
devres_release_all+0xb8/0x110 device_unbind_cleanup+0xe/0x70 device_release_driver_internal+0x1d2/0x210 bus_remove_device+0xd7/0x150 device_del+0x155/0x3e0 ? lock_release+0x142/0x290 cdev_device_del+0x15/0x50 cxl_memdev_unregister+0x54/0x70 [cxl_core]
This crash is due to the clearing out the cxl_memdev's driver context (@cxlds) before the subsystem is done with it. This is ultimately due to the region(s), that this memdev is a member, being torn down and expecting to be able to de-reference @cxlds, like here:
static int cxl_region_decode_reset(struct cxl_region *cxlr, int count) ... if (cxlds->rcd) goto endpoint_reset; ...
Fix it by keeping the driver context valid until memdev-device unregistration, and subsequently the entire stack of related dependencies, unwinds.
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Analysis
by VulDB Data Team • 12/30/2024
The vulnerability CVE-2023-52849 represents a critical kernel NULL pointer dereference issue within the Linux kernel's Compute Express Link (CXL) memory subsystem. This flaw manifests specifically during the shutdown sequence when the cxl_mock_mem device is removed, creating a race condition between device cleanup operations and subsystem references to critical driver context structures. The issue stems from improper ordering in the device destruction process where the driver context (@cxlds) is prematurely cleared before all dependent subsystem components have completed their operations, leading to a kernel panic and system crash.
The technical root cause involves a fundamental flaw in the device teardown sequence within the cxl_core kernel module. When a CXL memory device is unregistered, the system attempts to clean up the driver context structure before all dependent regions and endpoints have finished processing their cleanup operations. This violates the expected dependency chain where subsystem components like cxl_region_decode_reset function rely on the existence of cxlds to properly handle region reset operations. The crash trace demonstrates a clear sequence where the kernel attempts to access cxlds->rcd field at address 0x44, which has already been freed or invalidated during the premature cleanup phase.
This vulnerability directly maps to CWE-476 Null Pointer Dereference, which occurs when a null pointer is dereferenced in a program, leading to system crashes or potential privilege escalation. The operational impact extends beyond simple system instability as it affects the reliability of CXL memory subsystems in data center environments where memory devices are frequently hot-plugged or reconfigured. The flaw particularly impacts systems using CXL memory controllers, including those in high-performance computing clusters, enterprise servers, and storage arrays that depend on the CXL specification for memory expansion and persistent memory capabilities.
The fix implemented addresses the shutdown ordering issue by ensuring that driver context structures remain valid until the complete device unregistration process is finished. This approach aligns with ATT&CK technique T1489, which involves system shutdown or restart to disrupt services, though in this case it's a protective measure rather than an attack vector. The solution involves maintaining the integrity of the @cxlds context throughout the entire teardown stack, preventing premature memory deallocation that would otherwise cause subsystem components to reference freed memory. This mitigation strategy ensures proper resource lifecycle management and prevents the kernel from accessing invalid memory addresses during device removal operations, thereby maintaining system stability and preventing potential denial of service conditions in production environments.
The vulnerability represents a significant risk to enterprise systems relying on CXL memory technologies, particularly in scenarios involving frequent device hot-plugging or dynamic memory allocation. The fix ensures that device cleanup operations follow proper dependency ordering, preventing the NULL pointer dereference that would otherwise occur during normal device removal sequences. This correction maintains the integrity of the CXL subsystem and prevents potential system crashes that could impact data center operations, virtualization platforms, and memory-intensive applications that depend on reliable CXL memory device management.