On 1/18/22 3:05 PM, Reinette Chatre wrote:
> The machine check recovery handling in SGX added the changes
> listed below to the freeing of pages in sgx_free_epc_page().
> The SGX reclaimer contains an open coded version of
> sgx_free_epc_page() and thus did not obtain the changes in
> support of poison handling.
I was trying to decide if this is an urgent fix or not. A more crisp
problem statement might have helped in the changelog.
But, from what I can tell, the most probable troublesome scenario here
would be something like:
1. Machine check (#MC) occurs (asynchronous, !MF_ACTION_REQUIRED)
2. arch_memory_failure() called is eventually
3. (SGX) page->poison set to 1
4. Page is reclaimed
5. Page added to normal free lists by sgx_reclaim_pages()
^ This is the bug
6. Page is reallocated by some innocent enclave, a second (synchronous)
in-kernel #MC is induced, probably during EADD instruction.
^ This is the fallout from the bug
#6 is unfortunate and can be avoided if this patch is applied.
Basically, this patch ensures that a bad enclave page is isolated
quickly and causes a minimal amount of collateral damage. Is this a
valid summary?
The SGX reclaimer code lacks page poison handling in its free
path. This can lead to completely avoidable machine checks if a
poisoned page is freed and reallocated instead of being
isolated.
