CVE-2021-47268 in Linux
Summary
by MITRE • 05/21/2024
In the Linux kernel, the following vulnerability has been resolved:
usb: typec: tcpm: cancel vdm and state machine hrtimer when unregister tcpm port
A pending hrtimer may expire after the kthread_worker of tcpm port is destroyed, see below kernel dump when do module unload, fix it by cancel the 2 hrtimers.
[ 111.517018] Unable to handle kernel paging request at virtual address ffff8000118cb880
[ 111.518786] blk_update_request: I/O error, dev sda, sector 60061185 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
[ 111.526594] Mem abort info:
[ 111.526597] ESR = 0x96000047
[ 111.526600] EC = 0x25: DABT (current EL), IL = 32 bits
[ 111.526604] SET = 0, FnV = 0
[ 111.526607] EA = 0, S1PTW = 0
[ 111.526610] Data abort info:
[ 111.526612] ISV = 0, ISS = 0x00000047
[ 111.526615] CM = 0, WnR = 1
[ 111.526619] swapper pgtable: 4k pages, 48-bit VAs, pgdp=0000000041d75000
[ 111.526623] [ffff8000118cb880] pgd=10000001bffff003, p4d=10000001bffff003, pud=10000001bfffe003, pmd=10000001bfffa003, pte=0000000000000000
[ 111.526642] Internal error: Oops: 96000047 [#1] PREEMPT SMP
[ 111.526647] Modules linked in: dwc3_imx8mp dwc3 phy_fsl_imx8mq_usb [last unloaded: tcpci]
[ 111.526663] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.13.0-rc4-00927-gebbe9dbd802c-dirty #36
[ 111.526670] Hardware name: NXP i.MX8MPlus EVK board (DT)
[ 111.526674] pstate: 800000c5 (Nzcv daIF -PAN -UAO -TCO BTYPE=--)
[ 111.526681] pc : queued_spin_lock_slowpath+0x1a0/0x390
[ 111.526695] lr : _raw_spin_lock_irqsave+0x88/0xb4
[ 111.526703] sp : ffff800010003e20
[ 111.526706] x29: ffff800010003e20 x28: ffff00017f380180
[ 111.537156] buffer_io_error: 6 callbacks suppressed
[ 111.537162] Buffer I/O error on dev sda1, logical block 60040704, async page read
[ 111.539932] x27: ffff00017f3801c0
[ 111.539938] x26: ffff800010ba2490 x25: 0000000000000000 x24: 0000000000000001
[ 111.543025] blk_update_request: I/O error, dev sda, sector 60061186 op 0x0:(READ) flags 0x0 phys_seg 7 prio class 0
[ 111.548304]
[ 111.548306] x23: 00000000000000c0 x22: ffff0000c2a9f184 x21: ffff00017f380180
[ 111.551374] Buffer I/O error on dev sda1, logical block 60040705, async page read
[ 111.554499]
[ 111.554503] x20: ffff0000c5f14210 x19: 00000000000000c0 x18: 0000000000000000
[ 111.557391] Buffer I/O error on dev sda1, logical block 60040706, async page read
[ 111.561218]
[ 111.561222] x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
[ 111.564205] Buffer I/O error on dev sda1, logical block 60040707, async page read
[ 111.570887] x14: 00000000000000f5 x13: 0000000000000001 x12: 0000000000000040
[ 111.570902] x11: ffff0000c05ac6d8
[ 111.583420] Buffer I/O error on dev sda1, logical block 60040708, async page read
[ 111.588978] x10: 0000000000000000 x9 : 0000000000040000
[ 111.588988] x8 : 0000000000000000
[ 111.597173] Buffer I/O error on dev sda1, logical block 60040709, async page read
[ 111.605766] x7 : ffff00017f384880 x6 : ffff8000118cb880
[ 111.605777] x5 : ffff00017f384880
[ 111.611094] Buffer I/O error on dev sda1, logical block 60040710, async page read
[ 111.617086] x4 : 0000000000000000 x3 : ffff0000c2a9f184
[ 111.617096] x2 : ffff8000118cb880
[ 111.622242] Buffer I/O error on dev sda1, logical block 60040711, async page read
[ 111.626927] x1 : ffff8000118cb880 x0 : ffff00017f384888
[ 111.626938] Call trace:
[ 111.626942] queued_spin_lock_slowpath+0x1a0/0x390
[ 111.795809] kthread_queue_work+0x30/0xc0
[ 111.799828] state_machine_timer_handler+0x20/0x30
[ 111.804624] __hrtimer_run_queues+0x140/0x1e0
[ 111.808990] hrtimer_interrupt+0xec/0x2c0
[ 111.813004] arch_timer_handler_phys+0x38/0x50
[ 111.817456] handle_percpu_devid_irq+0x88/0x150
[ 111.821991] __handle_domain_irq+0x80/0xe0
[ 111.826093] gic_handle_irq+0xc0/0x140
[ 111.829848] el1_irq+0xbc/0x154
[ 111.832991] arch_cpu_idle+0x1c/0x2c
[ 111.836572] default_idle_call+0x24/0x6c
[ 111.840497] do_idle+0x238/0x2ac
[ 1
---truncated---
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Analysis
by VulDB Data Team • 12/26/2024
The vulnerability identified as CVE-2021-47268 resides within the Linux kernel's USB Type-C implementation, specifically within the Thunderbolt Control Protocol Manager (TCPM) subsystem. This flaw manifests during the unloading of USB Type-C modules when the kernel attempts to clean up resources associated with the TCPM port. The issue stems from the improper handling of high-resolution timers (hrtimers) that are scheduled to execute within the TCPM's kthread worker context. When a module is unloaded, the kthread worker is destroyed, but pending hrtimers may still be scheduled to execute, leading to a race condition that can cause kernel memory access violations.
The technical flaw occurs when the TCPM port's hrtimers are not properly canceled before the associated kthread worker is destroyed. These timers, which manage the state machine and VDM (Vendor Defined Messages) processing, can trigger execution after the memory structures they reference have already been freed. This creates a scenario where the kernel attempts to access virtual memory addresses that are no longer valid, resulting in a kernel paging fault as evidenced in the provided kernel dump. The error trace shows the system attempting to handle a memory access violation when the timer handler executes, specifically referencing the queued_spin_lock_slowpath function, which indicates a failed attempt to acquire a lock on freed memory structures.
The operational impact of this vulnerability is significant, as it can lead to kernel oops and system instability during module unloading operations. The described kernel dump shows the system attempting to access memory at address ffff8000118cb880, which is clearly outside the valid kernel address space, resulting in I/O errors and potential system crashes. This vulnerability affects systems running Linux kernels where USB Type-C functionality is utilized, particularly those implementing the TCPM subsystem such as embedded systems using i.MX8MPlus processors. The flaw can be exploited by an attacker who has the ability to unload USB Type-C modules, potentially causing denial of service or system instability.
Mitigation strategies for this vulnerability involve ensuring that all hrtimers associated with a TCPM port are properly canceled before the kthread worker is destroyed during module unloading. The fix implemented in the kernel patch ensures that both the VDM and state machine hrtimers are explicitly canceled before the TCPM port is unregistered. This aligns with the principles outlined in CWE-459, which addresses incomplete cleanup of resources, and follows the ATT&CK technique T1490 for Deobfuscation of Files or Information, though in this case the issue is more about proper resource management. Organizations should update to kernel versions containing the fix, typically kernel versions 5.14 and later, to prevent potential exploitation. Additionally, system administrators should monitor for any kernel oops or memory access violations during module loading and unloading operations as early indicators of this vulnerability. The fix demonstrates proper kernel resource management practices and follows the principle of ensuring that all asynchronous operations are properly terminated before resource cleanup begins.