Re: [PATCH V2] x86/sgx: Add poison handling to reclaimer

[Jarkko Sakkinen <jarkko@kernel.org>]

On Sun, Jan 23, 2022 at 01:27:10AM +0200, Jarkko Sakkinen wrote:
> On Thu, Jan 20, 2022 at 10:20:01AM -0800, Reinette Chatre wrote:
> > Hi Jarkko,
> > 
> > On 1/20/2022 5:09 AM, Jarkko Sakkinen wrote:
> > > On Wed, 2022-01-19 at 14:23 -0800, Reinette Chatre wrote:
> > >> The SGX reclaimer code lacks page poison handling in its main
> > >> free path. This can lead to avoidable machine checks if a
> > >> poisoned page is freed and reallocated instead of being
> > >> isolated.
> > >>
> > >> A troublesome scenario is:
> > >>  1. Machine check (#MC) occurs (asynchronous, !MF_ACTION_REQUIRED)
> > >>  2. arch_memory_failure() is eventually called
> > >>  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 (poison pages should be isolated on the
> > >>     sgx_poison_page_list instead)
> > >>  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 by replacing the open coded
> > >> enclave page freeing code in the reclaimer with sgx_free_epc_page()
> > >> to obtain support for poison page handling that includes placing the
> > >> poisoned page on the correct list.
> > >>
> > >> Fixes: d6d261bded8a ("x86/sgx: Add new sgx_epc_page flag bit to mark
> > >> free pages")
> > >> Fixes: 992801ae9243 ("x86/sgx: Initial poison handling for dirty and
> > >> free pages")
> > > 
> > > Same comment as for the first version: remove the first fixes tag.
> > > 
> > 
> > For completeness I'll duplicate my response also:
> > 
> > The commit you refer to, commit d6d261bded8a ("x86/sgx: Add new
> > sgx_epc_page flag bit to mark free pages", introduced a new page flag bit
> > (SGX_EPC_PAGE_IS_FREE) that should be set when an EPC page is freed. The
> > commit also sets the bit in sgx_free_epc_page() when an EPC page is freed.
> > The commit should also have set that bit when the EPC page is freed in the
> > reclaimer, which contains an open coded version of sgx_free_epc_page(),
> > but it did not. This fix adds the snippet that was omitted from that
> > commit.
> > 
> > Reinette
> 
> Let me check. Yes, I can see it in lore, but somehow it has slipped out
> of my inbox. My apologies.

And thanks for re-elaborating this.

/Jarkko