[PATCH v2 1/8] x86/sev: Make SVSM calls preemption-safe

[Melody Wang]

Two functions in the the SVSM vTPM guest implementation do not disable
preemption when fetching the SVSM Calling Area Address (CAA).

The SVSM CAA is a per-CPU structure. When a thread is preempted and
migrated to a different CPU after fetching the per-CPU CAA, the SVSM
call will execute on the new CPU with the original CPU's CAA. Which is
wrong.

Move the CAA fetching operation inside svsm_perform_call_protocol()
which disables interrupts around the SVSM call and thus runs
preemption-safe.

Fixes: 770de678bc28 ("x86/sev: Add SVSM vTPM probe/send_command functions")
Signed-off-by: Melody Wang <huibo.wang@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 arch/x86/coco/sev/svsm.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/coco/sev/svsm.c b/arch/x86/coco/sev/svsm.c
index 916d62cd17dc..2d493d55ff2e 100644
--- a/arch/x86/coco/sev/svsm.c
+++ b/arch/x86/coco/sev/svsm.c
@@ -74,6 +74,14 @@ int svsm_perform_call_protocol(struct svsm_call *call)
 
 	flags = native_local_irq_save();
 
+	/*
+	 * 'caa' is a per-CPU variable. To avoid using a stale or incorrect
+	 * 'caa' if the task is preempted or migrated to another CPU after it
+	 * is fetched, always fetch 'caa' and then issue the SVSM call with
+	 * interrupts disabled. This ensures the correct 'caa' is used.
+	 */
+	call->caa = svsm_get_caa();
+
 	ghcb = __sev_get_ghcb(&state);
 
 	do {
@@ -321,7 +329,6 @@ int snp_svsm_vtpm_send_command(u8 *buffer)
 {
 	struct svsm_call call = {};
 
-	call.caa = svsm_get_caa();
 	call.rax = SVSM_VTPM_CALL(SVSM_VTPM_CMD);
 	call.rcx = __pa(buffer);
 
@@ -345,7 +352,6 @@ bool snp_svsm_vtpm_probe(void)
 	if (!snp_vmpl)
 		return false;
 
-	call.caa = svsm_get_caa();
 	call.rax = SVSM_VTPM_CALL(SVSM_VTPM_QUERY);
 
 	if (svsm_perform_call_protocol(&call))
-- 
2.43.0