CVE-2021-47169 in Linux
Summary
by MITRE • 03/25/2024
In the Linux kernel, the following vulnerability has been resolved:
serial: rp2: use 'request_firmware' instead of 'request_firmware_nowait'
In 'rp2_probe', the driver registers 'rp2_uart_interrupt' then calls 'rp2_fw_cb' through 'request_firmware_nowait'. In 'rp2_fw_cb', if the firmware don't exists, function just return without initializing ports of 'rp2_card'. But now the interrupt handler function has been registered, and when an interrupt comes, 'rp2_uart_interrupt' may access those ports then causing NULL pointer dereference or other bugs.
Because the driver does some initialization work in 'rp2_fw_cb', in order to make the driver ready to handle interrupts, 'request_firmware' should be used instead of asynchronous 'request_firmware_nowait'.
This report reveals it:
INFO: trying to register non-static key. the code is fine but needs lockdep annotation. turning off the locking correctness validator. CPU: 2 PID: 0 Comm: swapper/2 Not tainted 4.19.177-gdba4159c14ef-dirty #45 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.0-59- gc9ba5276e321-prebuilt.qemu.org 04/01/2014 Call Trace: __dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0xec/0x156 lib/dump_stack.c:118 assign_lock_key kernel/locking/lockdep.c:727 [inline]
register_lock_class+0x14e5/0x1ba0 kernel/locking/lockdep.c:753 __lock_acquire+0x187/0x3750 kernel/locking/lockdep.c:3303 lock_acquire+0x124/0x340 kernel/locking/lockdep.c:3907 __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
_raw_spin_lock+0x32/0x50 kernel/locking/spinlock.c:144 spin_lock include/linux/spinlock.h:329 [inline]
rp2_ch_interrupt drivers/tty/serial/rp2.c:466 [inline]
rp2_asic_interrupt.isra.9+0x15d/0x990 drivers/tty/serial/rp2.c:493 rp2_uart_interrupt+0x49/0xe0 drivers/tty/serial/rp2.c:504 __handle_irq_event_percpu+0xfb/0x770 kernel/irq/handle.c:149 handle_irq_event_percpu+0x79/0x150 kernel/irq/handle.c:189 handle_irq_event+0xac/0x140 kernel/irq/handle.c:206 handle_fasteoi_irq+0x232/0x5c0 kernel/irq/chip.c:725 generic_handle_irq_desc include/linux/irqdesc.h:155 [inline]
handle_irq+0x230/0x3a0 arch/x86/kernel/irq_64.c:87 do_IRQ+0xa7/0x1e0 arch/x86/kernel/irq.c:247 common_interrupt+0xf/0xf arch/x86/entry/entry_64.S:670 RIP: 0010:native_safe_halt+0x28/0x30 arch/x86/include/asm/irqflags.h:61 Code: 00 00 55 be 04 00 00 00 48 c7 c7 00 c2 2f 8c 48 89 e5 e8 fb 31 e7 f8 8b 05 75 af 8d 03 85 c0 7e 07 0f 00 2d 8a 61 65 00 fb f4 c3 90 90 90 90 90 90 0f 1f 44 00 00 55 48 89 e5 41 57 41 56 41 RSP: 0018:ffff88806b71fcc8 EFLAGS: 00000246 ORIG_RAX: ffffffffffffffde RAX: 0000000000000000 RBX: ffffffff8bde7e48 RCX: ffffffff88a21285 RDX: 0000000000000000 RSI: 0000000000000004 RDI: ffffffff8c2fc200 RBP: ffff88806b71fcc8 R08: fffffbfff185f840 R09: fffffbfff185f840 R10: 0000000000000001 R11: fffffbfff185f840 R12: 0000000000000002 R13: ffffffff8bea18a0 R14: 0000000000000000 R15: 0000000000000000 arch_safe_halt arch/x86/include/asm/paravirt.h:94 [inline]
default_idle+0x6f/0x360 arch/x86/kernel/process.c:557 arch_cpu_idle+0xf/0x20 arch/x86/kernel/process.c:548 default_idle_call+0x3b/0x60 kernel/sched/idle.c:93 cpuidle_idle_call kernel/sched/idle.c:153 [inline]
do_idle+0x2ab/0x3c0 kernel/sched/idle.c:263 cpu_startup_entry+0xcb/0xe0 kernel/sched/idle.c:369 start_secondary+0x3b8/0x4e0 arch/x86/kernel/smpboot.c:271 secondary_startup_64+0xa4/0xb0 arch/x86/kernel/head_64.S:243 BUG: unable to handle kernel NULL pointer dereference at 0000000000000010 PGD 8000000056d27067 P4D 8000000056d27067 PUD 56d28067 PMD 0 Oops: 0000 [#1] PREEMPT SMP KASAN PTI
CPU: 2 PID: 0 Comm: swapper/2 Not tainted 4.19.177-gdba4159c14ef-dirty #45 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.0-59- gc9ba5276e321-prebuilt.qemu.org 04/01/2014 RIP: 0010:readl arch/x86/include/asm/io.h:59 [inline]
RIP: 0010:rp2_ch_interrupt drivers/tty/serial/rp2.c:472 [inline]
RIP: 0010:rp2_asic_interrupt.isra.9+0x181/0x990 drivers/tty/serial/rp2.c: 493 Co ---truncated---
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Analysis
by VulDB Data Team • 08/04/2025
The vulnerability identified as CVE-2021-47169 resides within the Linux kernel's serial driver implementation, specifically in the rp2 driver responsible for handling certain types of serial communication hardware. This flaw manifests due to improper firmware loading mechanisms that result in a race condition between driver initialization and interrupt handling. The issue stems from the use of an asynchronous firmware loading function, request_firmware_nowait, in the rp2_probe function, which registers an interrupt handler before the firmware loading completes. When the firmware fails to load, the driver does not properly initialize all necessary port structures, yet the interrupt handler has already been registered. This creates a scenario where an interrupt can trigger rp2_uart_interrupt, which attempts to access uninitialized port structures, leading to potential NULL pointer dereferences and system instability. The vulnerability is categorized under CWE-362, which deals with race conditions, and aligns with ATT&CK technique T1059.001, which involves executing malicious code through kernel-level operations. The kernel's locking correctness validator produces warnings indicating non-static key registration, highlighting the improper synchronization mechanisms. During exploitation, the system exhibits kernel oops and NULL pointer dereference errors, indicating memory access violations that can lead to system crashes or potential privilege escalation. The root cause lies in the driver's assumption that firmware loading will complete before interrupts are processed, which is not guaranteed with asynchronous loading methods. This vulnerability demonstrates the critical importance of proper initialization order and synchronization in kernel drivers, particularly when dealing with interrupt handlers and hardware resource management. The fix involves replacing the asynchronous request_firmware_nowait with the synchronous request_firmware function, ensuring that all necessary initialization occurs before the interrupt handler is registered. This change prevents the race condition by guaranteeing that firmware loading completes and port structures are initialized before any interrupt processing can occur, thereby maintaining system stability and preventing potential security implications. The issue underscores the necessity for robust error handling and proper state management in kernel-level drivers, where premature interrupt registration can lead to exploitable conditions. The reported stack trace shows the interrupt handler attempting to access memory locations that were never properly initialized, confirming the null pointer dereference scenario. This vulnerability affects systems using the rp2 serial driver and emphasizes the need for careful firmware loading procedures in kernel modules to prevent race conditions and maintain system integrity. The fix aligns with kernel development best practices for driver initialization and interrupt handling, ensuring that all resources are properly allocated and initialized before being made available for concurrent access.