Re: [PATCH v3 10/14] configfs-tsm: Allow the privlevel_floor attribute to be updated

[Tom Lendacky]

On 4/16/24 10:57, Dan Williams wrote:
> Tom Lendacky wrote:
> > On 4/15/24 23:55, Dan Williams wrote:
> > > Tom Lendacky wrote:
> > > > With the introduction of an SVSM, Linux will be running at a non-zero
> > > > VMPL. Any request for an attestation report at a higher priviledge VMPL
> > > > than what Linux is currently running will result in an error. Allow for
> > > > the privlevel_floor attribute to be updated dynamically so that the
> > > > attribute may be set dynamically.
> > > >
> > > > Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> > > > ---
> > > >    drivers/virt/coco/sev-guest/sev-guest.c | 5 ++++-
> > > >    include/linux/tsm.h                     | 2 +-
> > > >    2 files changed, 5 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
> > > > index 1ff897913bf4..bba6531cb606 100644
> > > > --- a/drivers/virt/coco/sev-guest/sev-guest.c
> > > > +++ b/drivers/virt/coco/sev-guest/sev-guest.c
> > > > @@ -885,7 +885,7 @@ static int sev_report_new(struct tsm_report *report, void *data)
> > > >    	return 0;
> > > >    }
> > > >    
> > > > -static const struct tsm_ops sev_tsm_ops = {
> > > > +static struct tsm_ops sev_tsm_ops = {
> > > >    	.name = KBUILD_MODNAME,
> > > >    	.report_new = sev_report_new,
> > > >    };
> > > > @@ -972,6 +972,9 @@ static int __init sev_guest_probe(struct platform_device *pdev)
> > > >    	snp_dev->input.resp_gpa = __pa(snp_dev->response);
> > > >    	snp_dev->input.data_gpa = __pa(snp_dev->certs_data);
> > > >    
> > > > +	/* Set the privlevel_floor attribute based on the current VMPL */
> > > > +	sev_tsm_ops.privlevel_floor = snp_get_vmpl();
> > >
> > > Why is this not vmpck_id?
> >
> > Good catch, this probably should be pulled out separately and submitted
> > as a Fixes: against the current support. If you think it's important
> > enough, I can do that and put this at the beginning of the series. Or I
> > can just modify this to use the vmpck_id value. Any preference?
>
> I dunno, you tell me. What breaks if privlevel_floor is mismatched vs
> vmpl and/or vmpck_id? If it warrants a "Fixes:" it should probably be
> broken out.
>
> However, I *guess* it is just adding some sanity checking precision to
> userspace requests and makes some input validation not catch errors when
> userspace tries to generate reports from the wrong level, right? I.e.
> privlevel_floor may be lower than expected, but userspace should not be
> depending on that since the report generation will fail.

Yeah, it just results in a different type of error. If the VMPL 
specified by the user is numerically lower than the vmpck_id, then the 
request will fail with a specific return code value. With the change to 
privlevel_floor to use vmpck_id, then you would just get the error that 
much sooner when trying to set a value that is lower than the floor.

Since I don't think the vmpck_id module parameter is a common case 
today, let's just leave that change in this patch.

Thanks,
Tom