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    SubjectRe: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP from userspace)
    On Tue, Jun 23, 2020 at 05:23:26PM +0200, Peter Zijlstra wrote:
    > On Tue, Jun 23, 2020 at 04:59:14PM +0200, Joerg Roedel wrote:
    > > On Tue, Jun 23, 2020 at 04:53:44PM +0200, Peter Zijlstra wrote:
    > > > +noinstr void idtentry_validate_ist(struct pt_regs *regs)
    > > > +{
    > > > + if ((regs->sp & ~(EXCEPTION_STKSZ-1)) ==
    > > > + (_RET_IP_ & ~(EXCEPTION_STKSZ-1)))
    > > > + die("IST stack recursion", regs, 0);
    > > > +}
    > >
    > > 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.

    Not with the current code, but possibly with SNP #VC exceptions:

    -> First #VC
    -> NMI before VC handler switched off its IST stack
    (now on NMI IST stack)
    -> Second SNP #VC exception before the NMI handler did the
    #VC stack check (because HV messed around with some pages
    touched there).

    In the second #VC you use the same IST stack as in the first #VC, but
    the the NMI-stack in-between.

    > Reliability of that depends on the unwinder, I wouldn't want the guess
    > uwinder to OOPS me by accident.

    It doesn't use the full unwinder, it just assumes that there is a
    pt_regs struct at the top of every kernel stack and walks through them
    until SP points to a user-space stack.

    As long as the assumption that there is a pt_regs struct on top of every
    stack holds, this should be safe. The assumption might be wrong when an
    exception happens during SYSCALL/SYSENTER entry, when the return frame
    is not written by hardware.


    Joerg

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