Race Condition Affecting kernel-rt-debug package, versions *

Note: Versions mentioned in the description apply only to the upstream kernel-rt-debug package and not the kernel-rt-debug package as distributed by Centos. See How to fix? for Centos:7 relevant fixed versions and status.

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.

• https://access.redhat.com/security/cve/CVE-2021-47069
• https://git.kernel.org/stable/c/4528c0c323085e645b8765913b4a7fd42cf49b65
• https://git.kernel.org/stable/c/807fa14536b26803b858da878b643be72952a097
• https://git.kernel.org/stable/c/a11ddb37bf367e6b5239b95ca759e5389bb46048