CVE-2022-48822 in Linux
Summary
by MITRE • 07/16/2024
In the Linux kernel, the following vulnerability has been resolved:
usb: f_fs: Fix use-after-free for epfile
Consider a case where ffs_func_eps_disable is called from ffs_func_disable as part of composition switch and at the same time ffs_epfile_release get called from userspace. ffs_epfile_release will free up the read buffer and call ffs_data_closed which in turn destroys ffs->epfiles and mark it as NULL. While this was happening the driver has already initialized the local epfile in ffs_func_eps_disable which is now freed and waiting to acquire the spinlock. Once spinlock is acquired the driver proceeds with the stale value of epfile and tries to free the already freed read buffer causing use-after-free.
Following is the illustration of the race:
CPU1 CPU2
ffs_func_eps_disable epfiles (local copy) ffs_epfile_release ffs_data_closed if (last file closed) ffs_data_reset ffs_data_clear ffs_epfiles_destroy spin_lock dereference epfiles
Fix this races by taking epfiles local copy & assigning it under spinlock and if epfiles(local) is null then update it in ffs->epfiles then finally destroy it. Extending the scope further from the race, protecting the ep related structures, and concurrent accesses.
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Analysis
by VulDB Data Team • 07/16/2024
The vulnerability described in CVE-2022-48822 represents a critical use-after-free condition within the Linux kernel's USB function filesystem implementation. This flaw exists in the f_fs subsystem which provides a filesystem interface for USB gadget functions. The vulnerability manifests when concurrent operations occur during USB function composition switching, creating a race condition between the driver's internal cleanup process and user-space file descriptor closure operations. The issue specifically impacts the usb function filesystem subsystem and falls under the category of memory safety vulnerabilities that can lead to system instability or potential privilege escalation.
The technical flaw occurs in the interaction between ffs_func_eps_disable and ffs_epfile_release functions during concurrent execution paths. When ffs_func_disable triggers endpoint disable operations, it calls ffs_func_eps_disable which creates a local copy of the epfiles structure. Simultaneously, if userspace invokes ffs_epfile_release, this function frees the read buffer and calls ffs_data_closed which ultimately destroys the ffs->epfiles structure and sets it to NULL. The race condition emerges when the driver has already initialized the local epfile copy but the spinlock acquisition occurs after the epfiles structure has been freed, leading to a use-after-free scenario where the driver attempts to free memory that has already been deallocated. This vulnerability directly maps to CWE-416, which addresses Use After Free conditions, and demonstrates the classic race condition pattern that can compromise memory safety in concurrent systems.
The operational impact of this vulnerability extends beyond simple system crashes, potentially enabling privilege escalation attacks that could allow malicious users to execute arbitrary code with kernel privileges. The race condition occurs during USB composition switching operations when the kernel handles multiple concurrent access patterns, making this vulnerability particularly dangerous in environments where USB gadget functionality is actively utilized. Attackers could exploit this condition by carefully orchestrating timing sequences to trigger the race, potentially leading to complete system compromise. The vulnerability affects Linux kernel versions where the f_fs subsystem is implemented and could be particularly problematic in embedded systems or devices that rely heavily on USB gadget functionality for device operation.
Mitigation strategies for this vulnerability require careful implementation of proper locking mechanisms and memory management protocols. The fix involves implementing a more robust approach to handling epfiles during concurrent access by taking local copies of epfiles under spinlock protection and ensuring proper state management before destruction. This approach aligns with ATT&CK technique T1068, which involves exploiting local privilege escalation opportunities, and requires the kernel to properly serialize access to shared resources. System administrators should ensure immediate patching of affected kernel versions and monitor for any signs of exploitation attempts. The fix demonstrates the importance of proper concurrent access control in kernel space operations and emphasizes the need for thorough testing of race condition scenarios in USB subsystem implementations. Additional protective measures include implementing kernel memory protection mechanisms and monitoring for anomalous USB gadget behavior that might indicate exploitation attempts.