3.1 KiB
CVE-2025-38334
Description
In the Linux kernel, the following vulnerability has been resolved:x86/sgx: Prevent attempts to reclaim poisoned pagesTL;DR: SGX page reclaim touches the page to copy its contents tosecondary storage. SGX instructions do not gracefully handle machinechecks. Despite this, the existing SGX code will try to reclaim pagesthat it knows are poisoned. Avoid even trying to reclaim poisoned pages.The longer story:Pages used by an enclave only get epc_page->poison set inarch_memory_failure() but they currently stay on sgx_active_page_list untilsgx_encl_release(), with the SGX_EPC_PAGE_RECLAIMER_TRACKED flag untouched.epc_page->poison is not checked in the reclaimer logic meaning that, if otherconditions are met, an attempt will be made to reclaim an EPC page that waspoisoned. This is bad because 1. we don't want that page to end up addedto another enclave and 2. it is likely to cause one core to shut downand the kernel to panic.Specifically, reclaiming uses microcode operations including "EWB" whichaccesses the EPC page contents to encrypt and write them out to non-SGXmemory. Those operations cannot handle MCEs in their accesses other thanby putting the executing core into a special shutdown state (affectingboth threads with HT.) The kernel will subsequently panic on theremaining cores seeing the core didn't enter MCE handler(s) in time.Call sgx_unmark_page_reclaimable() to remove the affected EPC page fromsgx_active_page_list on memory error to stop it being considered forreclaiming.Testing epc_page->poison in sgx_reclaim_pages() would also work but I assumeit's better to add code in the less likely paths.The affected EPC page is not added to &node->sgx_poison_page_list untillater in sgx_encl_release()->sgx_free_epc_page() when it is EREMOVEd.Membership on other lists doesn't change to avoid changing any of thelists' semantics except for sgx_active_page_list. There's a "TBD" commentin arch_memory_failure() about pre-emptive actions, the goal here is notto address everything that it may imply.This also doesn't completely close the time window when a memory errornotification will be fatal (for a not previously poisoned EPC page) --the MCE can happen after sgx_reclaim_pages() has selected its candidatesor even inside a microcode operation (actually easy to trigger due tothe amount of time spent in them.)The spinlock in sgx_unmark_page_reclaimable() is safe becausememory_failure() runs in process context and no spinlocks are held,explicitly noted in a mm/memory-failure.c comment.
POC
Reference
No PoCs from references.