Description

In the Linux kernel, the following vulnerability has been resolved: x86/sgx: Prevent attempts to reclaim poisoned pages TL;DR: SGX page reclaim touches the page to copy its contents to secondary storage. SGX instructions do not gracefully handle machine checks. Despite this, the existing SGX code will try to reclaim pages that 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 in arch_memory_failure() but they currently stay on sgx_active_page_list until sgx_encl_release(), with the SGX_EPC_PAGE_RECLAIMER_TRACKED flag untouched. epc_page->poison is not checked in the reclaimer logic meaning that, if other conditions are met, an attempt will be made to reclaim an EPC page that was poisoned. This is bad because 1. we don't want that page to end up added to another enclave and 2. it is likely to cause one core to shut down and the kernel to panic. Specifically, reclaiming uses microcode operations including "EWB" which accesses the EPC page contents to encrypt and write them out to non-SGX memory. Those operations cannot handle MCEs in their accesses other than by putting the executing core into a special shutdown state (affecting both threads with HT.) The kernel will subsequently panic on the remaining cores seeing the core didn't enter MCE handler(s) in time. Call sgx_unmark_page_reclaimable() to remove the affected EPC page from sgx_active_page_list on memory error to stop it being considered for reclaiming. Testing epc_page->poison in sgx_reclaim_pages() would also work but I assume it's better to add code in the less likely paths. The affected EPC page is not added to &node->sgx_poison_page_list until later in sgx_encl_release()->sgx_free_epc_page() when it is EREMOVEd. Membership on other lists doesn't change to avoid changing any of the lists' semantics except for sgx_active_page_list. There's a "TBD" comment in arch_memory_failure() about pre-emptive actions, the goal here is not to address everything that it may imply. This also doesn't completely close the time window when a memory error notification will be fatal (for a not previously poisoned EPC page) -- the MCE can happen after sgx_reclaim_pages() has selected its candidates or even *inside* a microcode operation (actually easy to trigger due to the amount of time spent in them.) The spinlock in sgx_unmark_page_reclaimable() is safe because memory_failure() runs in process context and no spinlocks are held, explicitly noted in a mm/memory-failure.c comment.

Metrics

 

CVSS Version 4.0 CVSS Version 3.x CVSS Version 2.0

NVD enrichment efforts reference publicly available information to associate vector strings. CVSS information contributed by other sources is also displayed.

CVSS 4.0 Severity and Vector Strings:

NIST: NVD

N/A

NVD assessment not yet provided.

CVSS 3.x Severity and Vector Strings:

NIST: NVD

Base Score:  N/A

NVD assessment not yet provided.

CVSS 2.0 Severity and Vector Strings:

NIST: NVD

Base Score: N/A

NVD assessment not yet provided.

In the Linux kernel, the following vulnerability has been resolved:

x86/sgx: Prevent attempts to reclaim poisoned pages

TL;DR: SGX page reclaim touches the page to copy its contents to
secondary storage. SGX instructions do not gracefully handle machine
checks. Despite this, the existing SGX code will try to reclaim pages
that 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 in
arch_memory_failure() but they currently stay on sgx_active_page_list until
sgx_encl_release(), with the SGX_EPC_PAGE_RECLAIMER_TRACKED flag untouched.

epc_page->poison is not checked in the reclaimer logic meaning that, if other
conditions are met, an attempt will be made to reclaim an EPC page that was
poisoned.  This is bad because 1. we don't want that page to end up added
to another enclave and 2. it is likely to cause one core to shut down
and the kernel to panic.

Specifically, reclaiming uses microcode operations including "EWB" which
accesses the EPC page contents to encrypt and write them out to non-SGX
memory.  Those operations cannot handle MCEs in their accesses other than
by putting the executing core into a special shutdown state (affecting
both threads with HT.)  The kernel will subsequently panic on the
remaining cores seeing the core didn't enter MCE handler(s) in time.

Call sgx_unmark_page_reclaimable() to remove the affected EPC page from
sgx_active_page_list on memory error to stop it being considered for
reclaiming.

Testing epc_page->poison in sgx_reclaim_pages() would also work but I assume
it's better to add code in the less likely paths.

The affected EPC page is not added to &node->sgx_poison_page_list until
later in sgx_encl_release()->sgx_free_epc_page() when it is EREMOVEd.
Membership on other lists doesn't change to avoid changing any of the
lists' semantics except for sgx_active_page_list.  There's a "TBD" comment
in arch_memory_failure() about pre-emptive actions, the goal here is not
to address everything th

https://git.kernel.org/stable/c/00a88e9ea1b170d579c56327c38f7e8cf689df87

https://git.kernel.org/stable/c/31dcbac94bfeabb86bf85b0c36803fdd6536437b

https://git.kernel.org/stable/c/62b62a2a6dc51ed6e8e334861f04220c9cf8106a

https://git.kernel.org/stable/c/dc5de5bd6deabd327ced2b2b1d0b4f14cd146afe

https://git.kernel.org/stable/c/ed16618c380c32c68c06186d0ccbb0d5e0586e59