On Tue, Apr 30, 2019 at 09:44:09AM -0700, Andy Lutomirski wrote:

On Sat, Apr 27, 2019 at 10:47 PM Mike Rapoport [off-list ref] wrote:

quoted

On Fri, Apr 26, 2019 at 09:42:23AM +0200, Peter Zijlstra wrote:

quoted

On Fri, Apr 26, 2019 at 12:45:52AM +0300, Mike Rapoport wrote:

quoted

If a system call runs in isolated context, it's accesses to kernel code and
data will be verified by SCI susbsytem.

Signed-off-by: Mike Rapoport <redacted>
---
 arch/x86/mm/fault.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

There's a distinct lack of touching do_double_fault(). It appears to me
that you'll instantly trigger #DF when you #PF, because the #PF handler
itself will not be able to run.

The #PF handler is able to run. On interrupt/error entry the cr3 is
switched to the full kernel page tables, pretty much like PTI does for
user <-> kernel transitions. It's in the patch 3.

PeterZ meant page_fault, not do_page_fault.  In your patch, page_fault
and some of error_entry run before that magic switchover happens.  If
they're not in the page tables, you double-fault.

The entry code is in sci page tables, just like in user-space page tables
with PTI.
 

And don't even try to do SCI magic in the double-fault handler.  As I
understand it, the SDM and APM aren't kidding when they say that #DF
is an abort, not a fault.  There is a single case in the kernel where
we recover from #DF, and it was vetted by microcode people.

-- 
Sincerely yours,
Mike.