CVE-2024-36936 Information

Description

In the Linux kernel the following vulnerability has been resolved:

efi/unaccepted: touch soft lockup during memory accept

Commit 50e782a86c98 (fi/unaccepted: Fix soft lockups caused by parallel memory acceptance) has released the spinlock so other CPUs can do memory acceptance in parallel and not triggers softlockup on other CPUs.

However the softlock up was intermittent shown up if the memory of the TD guest is large and the timeout of softlockup is set to 1 second:

RIP: 0010:_raw_spin_unlock_irqrestore Call Trace: ? __hrtimer_run_queues ? hrtimer_interrupt ? watchdog_timer_fn ? __sysvec_apic_timer_interrupt ? __pfx_watchdog_timer_fn ? sysvec_apic_timer_interrupt ? __hrtimer_run_queues ? hrtimer_interrupt ? asm_sysvec_apic_timer_interrupt ? _raw_spin_unlock_irqrestore ? __sysvec_apic_timer_interrupt ? sysvec_apic_timer_interrupt accept_memory try_to_accept_memory do_huge_pmd_anonymous_page get_page_from_freelist __handle_mm_fault __alloc_pages __folio_alloc ? __tdx_hypercall handle_mm_fault vma_alloc_folio do_user_addr_fault do_huge_pmd_anonymous_page exc_page_fault ? __do_huge_pmd_anonymous_page asm_exc_page_fault __handle_mm_fault

When the local irq is enabled at the end of accept_memory() the softlockup detects that the watchdog on single CPU has not been fed for a while. That is to say even other CPUs will not be blocked by spinlock the current CPU might be stunk with local irq disabled for a while which hurts not only nmi watchdog but also softlockup.

Chao Gao pointed out that the memory accept could be time costly and there was similar report before. Thus to avoid any softlocup detection during this stage give the softlockup a flag to skip the timeout check at the end of accept_memory() by invoking touch_softlockup_watchdog().

Reference

https://git.kernel.org/stable/c/781e34b736014188ba9e46a71535237313dcda81 https://git.kernel.org/stable/c/1c5a1627f48105cbab81d25ec2f72232bfaa8185