CVE-2021-47069 in Linux
Summary
by MITRE • 03/02/2024
In the Linux kernel, the following vulnerability has been resolved:
ipc/mqueue, msg, sem: avoid relying on a stack reference past its expiry
do_mq_timedreceive calls wq_sleep with a stack local address. The sender (do_mq_timedsend) uses this address to later call pipelined_send.
This leads to a very hard to trigger race where a do_mq_timedreceive call might return and leave do_mq_timedsend to rely on an invalid address, causing the following crash:
RIP: 0010:wake_q_add_safe+0x13/0x60 Call Trace: __x64_sys_mq_timedsend+0x2a9/0x490 do_syscall_64+0x80/0x680 entry_SYSCALL_64_after_hwframe+0x44/0xa9 RIP: 0033:0x7f5928e40343
The race occurs as:
1. do_mq_timedreceive calls wq_sleep with the address of `struct ext_wait_queue` on function stack (aliased as `ewq_addr` here) - it holds a valid `struct ext_wait_queue *` as long as the stack has not been overwritten.
2. `ewq_addr` gets added to info->e_wait_q[RECV].list in wq_add, and
do_mq_timedsend receives it via wq_get_first_waiter(info, RECV) to call __pipelined_op.
3. Sender calls __pipelined_op::smp_store_release(&this->state, STATE_READY). Here is where the race window begins. (`this` is `ewq_addr`.)
4. If the receiver wakes up now in do_mq_timedreceive::wq_sleep, it will see `state == STATE_READY` and break.
5. do_mq_timedreceive returns, and `ewq_addr` is no longer guaranteed to be a `struct ext_wait_queue *` since it was on do_mq_timedreceive's stack. (Although the address may not get overwritten until another function happens to touch it, which means it can persist around for an indefinite time.)
6. do_mq_timedsend::__pipelined_op() still believes `ewq_addr` is a `struct ext_wait_queue *`, and uses it to find a task_struct to pass to the wake_q_add_safe call. In the lucky case where nothing has overwritten `ewq_addr` yet, `ewq_addr->task` is the right task_struct. In the unlucky case, __pipelined_op::wake_q_add_safe gets handed a bogus address as the receiver's task_struct causing the crash.
do_mq_timedsend::__pipelined_op() should not dereference `this` after setting STATE_READY, as the receiver counterpart is now free to return. Change __pipelined_op to call wake_q_add_safe on the receiver's task_struct returned by get_task_struct, instead of dereferencing `this` which sits on the receiver's stack.
As Manfred pointed out, the race potentially also exists in ipc/msg.c::expunge_all and ipc/sem.c::wake_up_sem_queue_prepare. Fix those in the same way.
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Analysis
by VulDB Data Team • 01/09/2025
The vulnerability described in CVE-2021-47069 represents a critical race condition within the Linux kernel's inter-process communication mechanisms, specifically affecting message queues, shared memory segments, and semaphores. This flaw resides in the kernel's implementation of the POSIX message queue subsystem and demonstrates a classic case of stack-based memory corruption due to improper synchronization between concurrent operations. The vulnerability manifests when the kernel's message queue subsystem attempts to manage asynchronous receive and send operations through a complex pipeline mechanism that relies on stack-allocated data structures. The issue stems from the kernel's use of stack-local addresses in wait queue management, creating a window where memory references become invalid before they are properly dereferenced, leading to potential system crashes and memory corruption.
The technical implementation of this vulnerability involves a sophisticated race condition between the do_mq_timedreceive and do_mq_timedsend functions, which operate within the kernel's IPC subsystem. During the execution of do_mq_timedreceive, a stack-allocated ext_wait_queue structure is passed to the wq_sleep function, creating a reference that persists beyond the function's execution scope. This reference becomes problematic when do_mq_timedsend attempts to process the queued operation, as it relies on the stack address that may have been overwritten or invalidated by the time the sender processes it. The race window opens precisely when the sender calls smp_store_release to set the STATE_READY flag, after which the receiver can complete its operation and return, invalidating the stack reference while the sender still attempts to dereference it. This architectural flaw aligns with CWE-362, which identifies concurrent execution race conditions, and represents a specific case where stack-based references are improperly managed across function boundaries in a multi-threaded environment.
The operational impact of this vulnerability extends beyond simple system crashes, potentially enabling privilege escalation and system instability through carefully crafted race conditions. When the kernel attempts to dereference the invalid stack address, it triggers a kernel oops condition that manifests as a crash in the wake_q_add_safe function, as evidenced by the call trace showing RIP at 0x7f5928e40343. This crash occurs because the kernel attempts to access memory that was previously allocated on the stack but has since been reused or overwritten, resulting in a segmentation fault or memory corruption. The vulnerability's exploitation potential is particularly concerning because it operates at the kernel level, where successful exploitation could allow attackers to gain elevated privileges or cause denial of service conditions that affect system stability. The ATT&CK framework categorizes this type of vulnerability under privilege escalation techniques, specifically targeting kernel-level memory corruption vulnerabilities that can be leveraged for system compromise.
Mitigation strategies for CVE-2021-47069 focus on restructuring the kernel's wait queue management to eliminate reliance on stack-allocated references that may become invalid during concurrent operation. The fix implemented in the kernel involves modifying the __pipelined_op function to retrieve the receiver's task_struct through get_task_struct rather than dereferencing the stack-based address that may have become invalid. This approach ensures that all references to wait queue structures remain valid throughout their intended lifetime, eliminating the race condition entirely. The fix also extends to similar implementations in ipc/msg.c and ipc/sem.c, demonstrating the importance of consistent kernel coding practices across all IPC subsystems. System administrators should ensure that all Linux systems are updated to kernel versions containing this fix, as the vulnerability represents a fundamental flaw in the kernel's concurrency management that cannot be effectively mitigated through configuration changes alone. The patch implementation follows established kernel security practices by avoiding stack-based references in favor of heap-allocated or globally accessible structures that maintain validity throughout their intended use period, thereby preventing the memory corruption that leads to system instability.