From: hshoexer <hshoexer@yerbouti.franken.de>
Subject: SEV-ES: ensure GHCB is cleared/invalidated after use
To: tech@openbsd.org
Date: Thu, 27 Nov 2025 15:46:19 +0100

Hi,

in SEV-ES mode, guest userland is allowed to execute the vmgexit
instruction, although it has no control over the GHCB.  Therefore,
it is important that the GHCB does not contain a valid request after
use.

In all "vmgexit paths" the GHCB is cleared by ghcb_sync_in() (it
calls ghcb_clear()) after returning from the hypervisor back into
the guest.

However, in _ghcb_mem_rw() I missed this when requesting MMIO writes
from the hypervisor.  The diff below corrects this.

I want to keep this pattern in all vmgexit paths:

	ghcb_sync_out
	vmgexit
	ghcb_verify_bm
	ghcb_sync_in

Therefore, I shuffled some code around instead of just calling
vmgexit_clear() in the else branch.

ok?  thoughts?

Take care,
HJ.

-----------------------------------------------------------------------
diff --git a/sys/arch/amd64/amd64/ghcb.c b/sys/arch/amd64/amd64/ghcb.c
index 2b0fa809570..b6aa2840228 100644
--- a/sys/arch/amd64/amd64/ghcb.c
+++ b/sys/arch/amd64/amd64/ghcb.c
@@ -271,7 +271,7 @@ _ghcb_mem_rw(vaddr_t addr, int valsz, void *val, bool read)
 	struct ghcb_sync	 syncout, syncin;
 	struct ghcb_sa		*ghcb;
 	unsigned long		 s;
-	struct ghcb_extra_regs	 ghcb_regs;
+	struct ghcb_extra_regs	 ghcb_regs, *pregs = NULL;
 
 	KASSERT(val != NULL);
 
@@ -334,15 +334,15 @@ _ghcb_mem_rw(vaddr_t addr, int valsz, void *val, bool read)
 		panic("invalid hypervisor response");
 	}
 
-	memset(&ghcb_regs, 0, sizeof(ghcb_regs));
-
 	if (read) {
+		memset(&ghcb_regs, 0, sizeof(ghcb_regs));
 		ghcb_regs.data = val;
 		ghcb_regs.data_sz = size;
-
-		ghcb_sync_in(NULL, &ghcb_regs, ghcb, &syncin);
+		pregs = &ghcb_regs;
 	}
 
+	ghcb_sync_in(NULL, pregs, ghcb, &syncin);
+
 	intr_restore(s);
 }