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

From: Tom Lendacky
Date: Tue Apr 16 2024 - 12:18:04 EST


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