On Tue, Jun 23, 2020 at 04:39:26PM +0100, Andrew Cooper wrote:

On 23/06/2020 16:23, Peter Zijlstra wrote:

quoted

On Tue, Jun 23, 2020 at 04:59:14PM +0200, Joerg Roedel wrote:

quoted

quoted

Yes, this is a start, it doesn't cover the case where the NMI stack is
in-between, so I think you need to walk down regs->sp too.

That shouldn't be possible with the current code, I think.

NMI; #MC; Anything which IRET but isn't fatal - #DB, or #BP from
patching, #GP from *_safe(), etc; NMI

Sure its a corner case, but did you hear that IST is evil?

Isn't current #MC unconditionally fatal from kernel? But yes, I was
sorta aware people want that changed.

And yes, NMI can recurse, mostly on #BP and #PF. Like I wrote, its
broken vs #MC.

But Joerg was talking about IST recursion with NMI in the middle,
something like: #DB, NMI, #DB, and not already being fatal. This one in
particular is ruled out by #DB itself clearing DR7 (but NMI would also
do that).

P.S. did you also hear that with Rowhammer, userspace has a nonzero
quantity of control over generating #MC, depending on how ECC is
configured on the platform.

Yes, excellent stuff.