Re: [Xen-devel] [PATCH] x86/HVM: correct notion of new CPL in task switch emulation

[Julien Grall <julien.grall@xxxxxxx>]

Hi Andrew,

On 02/06/17 21:02, Andrew Cooper wrote:
> On 01/06/17 13:11, Jan Beulich wrote:
> > Commit aac1df3d03 ("x86/HVM: introduce hvm_get_cpl() and respective
> > hook") went too far in one aspect: When emulating a task switch we
> > really shouldn't be looking at what hvm_get_cpl() returns, as we're
> > switching all segment registers.
> >
> > However, instead of reverting the relevant parts of that commit, have
> > the caller tell the segment loading function what the new CPL is. This
> > at once fixes ES being loaded before CS so far having had its checks
> > done against the old CPL.
> I'd have an extra paragraph describing the symptoms in practice.  e.g.
>
> This bug manifests as a vmentry failure for 32bit VMs which use task
> gates to service interrupts/exceptions, in situations where delivering
> the event interrupts user code, and a privilege increase is required.
>
> ?
>
> > Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> I have finally managed to reproduce the original vmentry failure with an
> XTF test.  This patch resolves the issue, so
>
> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Tested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>
> Julien: This really should be taken in 4.9, otherwise 32bit VMs will
> sporadically crash, especially windows which uses this mechanism to
> handle NMIs.
Release-acked-by: Julien Grall <julien.grall@xxxxxxx>

Cheers,

> > ---
> > An alternative to adding yet another parameter to the function would
> > be to have "cpl" have a special case value (e.g. negative) to indicate
> > VM86 mode. That would allow replacing the current "eflags" parameter.
> Keeping the parameters separate is clearer.  It is not like this is a
> hotpath we need to optimise.
>
> ~Andrew
--
Julien Grall

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