CVE-2022-48760 in Linux
Summary
by MITRE • 06/20/2024
In the Linux kernel, the following vulnerability has been resolved:
USB: core: Fix hang in usb_kill_urb by adding memory barriers
The syzbot fuzzer has identified a bug in which processes hang waiting for usb_kill_urb() to return. It turns out the issue is not unlinking the URB; that works just fine. Rather, the problem arises when the wakeup notification that the URB has completed is not received.
The reason is memory-access ordering on SMP systems. In outline form, usb_kill_urb() and __usb_hcd_giveback_urb() operating concurrently on different CPUs perform the following actions:
CPU 0 CPU 1 ---------------------------- --------------------------------- usb_kill_urb(): __usb_hcd_giveback_urb(): ... ... atomic_inc(&urb->reject); atomic_dec(&urb->use_count); ... ... wait_event(usb_kill_urb_queue, atomic_read(&urb->use_count) == 0); if (atomic_read(&urb->reject)) wake_up(&usb_kill_urb_queue);
Confining your attention to urb->reject and urb->use_count, you can see that the overall pattern of accesses on CPU 0 is:
write urb->reject, then read urb->use_count;
whereas the overall pattern of accesses on CPU 1 is:
write urb->use_count, then read urb->reject.
This pattern is referred to in memory-model circles as SB (for "Store Buffering"), and it is well known that without suitable enforcement of the desired order of accesses -- in the form of memory barriers -- it is entirely possible for one or both CPUs to execute their reads ahead of their writes. The end result will be that sometimes CPU 0 sees the old un-decremented value of urb->use_count while CPU 1 sees the old un-incremented value of urb->reject. Consequently CPU 0 ends up on the wait queue and never gets woken up, leading to the observed hang in usb_kill_urb().
The same pattern of accesses occurs in usb_poison_urb() and the failure pathway of usb_hcd_submit_urb().
The problem is fixed by adding suitable memory barriers. To provide proper memory-access ordering in the SB pattern, a full barrier is required on both CPUs. The atomic_inc() and atomic_dec() accesses themselves don't provide any memory ordering, but since they are present, we can use the optimized smp_mb__after_atomic() memory barrier in the various routines to obtain the desired effect.
This patch adds the necessary memory barriers.
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Analysis
by VulDB Data Team • 09/17/2025
The vulnerability described in CVE-2022-48760 represents a critical race condition affecting the Linux kernel's USB subsystem, specifically within the USB core driver. This issue manifests as a deadlock scenario during USB endpoint removal operations, where processes become indefinitely blocked waiting for the usb_kill_urb() function to complete. The problem stems from improper memory ordering on symmetric multiprocessing systems, creating a scenario where concurrent execution paths on different CPU cores fail to maintain the necessary synchronization guarantees for shared data structures. The vulnerability impacts the fundamental USB communication mechanisms that underpin device connectivity and data transfer operations across Linux systems.
The technical flaw occurs due to a specific memory access pattern that violates the expected ordering constraints between two atomic operations executing on different CPU cores. When usb_kill_urb() and __usb_hcd_giveback_urb() execute concurrently, they create a classic store buffering scenario where CPU 0 writes to urb->reject and then reads urb->use_count while CPU 1 writes to urb->use_count and then reads urb->reject. This SB (Store Buffering) pattern creates a race condition where memory operations can be reordered by the CPU's memory subsystem, causing each CPU to see stale values from the other's operations. The atomic_inc() and atomic_dec() operations themselves do not provide memory ordering guarantees, which means the underlying hardware's memory model allows for these reordering behaviors to occur without explicit synchronization.
The operational impact of this vulnerability is severe and affects all Linux systems running kernel versions containing the affected USB core code. Processes attempting to terminate USB operations hang indefinitely, leading to system instability, resource exhaustion, and potential denial of service conditions. The hang occurs specifically during USB device removal, driver unloading, or when attempting to cancel pending USB transfers, making it particularly problematic for systems with active USB device usage. This vulnerability can affect any system using USB devices, from desktop computers to embedded systems, and particularly impacts server environments where USB device management is critical. The issue is exacerbated in multi-core systems where the timing of CPU execution can trigger the race condition consistently.
The fix for CVE-2022-48760 involves implementing proper memory barriers to enforce the required ordering between the atomic operations on both CPU cores. The solution specifically employs smp_mb__after_atomic() memory barriers which provide the necessary synchronization without introducing unnecessary performance overhead. This approach addresses the root cause by ensuring that when one CPU writes to a shared data structure, the other CPU will see that write before proceeding with its own operations. The memory barriers are strategically placed in usb_kill_urb(), usb_poison_urb(), and the failure pathway of usb_hcd_submit_urb() functions to prevent the store buffering scenario that leads to the deadlock condition. This fix aligns with standard concurrency control practices and follows the established patterns for memory ordering in kernel-level concurrent programming, addressing the vulnerability at its fundamental cause rather than through workarounds or application-level fixes.
This vulnerability maps to CWE-1236, which specifically addresses improper handling of memory ordering in concurrent systems, and demonstrates behaviors consistent with ATT&CK technique T1489, which involves creating or manipulating system resources to cause denial of service. The memory ordering issue affects the kernel's ability to maintain consistent state across concurrent operations, which is fundamental to system stability and security. The fix ensures that the kernel's USB subsystem maintains proper synchronization guarantees, preventing both the immediate hang condition and potential indirect security implications that could arise from system instability. The resolution maintains backward compatibility while providing the necessary concurrency guarantees required for safe operation in multi-core environments.