x86/sgx: Resolves SECS reclaim vs. page fault for EAUG race

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

x86/sgx: Resolves SECS reclaim vs. page fault for EAUG race

The SGX EPC reclaimer (ksgxd) may reclaim the SECS EPC page for an

enclave and set secs.epc_page to NULL. The SECS page is used for EAUG

and ELDU in the SGX page fault handler. However, the NULL check for

secs.epc_page is only done for ELDU, not EAUG before being used.

Fix this by doing the same NULL check and reloading of the SECS page as

needed for both EAUG and ELDU.

The SECS page holds global enclave metadata. It can only be reclaimed

when there are no other enclave pages remaining. At that point,

virtually nothing can be done with the enclave until the SECS page is

paged back in.

An enclave can not run nor generate page faults without a resident SECS

page. But it is still possible for a #PF for a non-SECS page to race

with paging out the SECS page: when the last resident non-SECS page A

triggers a #PF in a non-resident page B, and then page A and the SECS

both are paged out before the #PF on B is handled.

Hitting this bug requires that race triggered with a #PF for EAUG.

Following is a trace when it happens.

BUG: kernel NULL pointer dereference, address: 0000000000000000

RIP: 0010:sgx_encl_eaug_page+0xc7/0x210

Call Trace:

? __kmem_cache_alloc_node+0x16a/0x440

? xa_load+0x6e/0xa0

sgx_vma_fault+0x119/0x230

__do_fault+0x36/0x140

do_fault+0x12f/0x400

__handle_mm_fault+0x728/0x1110

handle_mm_fault+0x105/0x310

do_user_addr_fault+0x1ee/0x750

? __this_cpu_preempt_check+0x13/0x20

exc_page_fault+0x76/0x180

asm_exc_page_fault+0x27/0x30