x86/sev: Disable MMIO emulation from user mode

A virt scenario can be constructed where MMIO memory can be user memory. When that happens, a race condition opens between when the hardware raises the #VC and when the #VC handler gets to emulate the instruction. If the MOVS is replaced with a MOVS accessing kernel memory in that small race window, then write to kernel memory happens as the access checks are not done at emulation time. Disable MMIO emulation in user mode temporarily until a sensible use case appears and justifies properly handling the race window. Fixes: 0118b60 ("x86/sev-es: Handle MMIO String Instructions") Reported-by: Tom Dohrmann <[email protected]> Signed-off-by: Borislav Petkov (AMD) <[email protected]> Tested-by: Tom Dohrmann <[email protected]> Cc: <[email protected]>
luigidematteis · Oct 9, 2023 · a37cd2a · a37cd2a
1 parent 8a749fd
commit a37cd2a
Showing 1 changed file with 3 additions and 0 deletions.
diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
@@ -1509,6 +1509,9 @@ static enum es_result vc_handle_mmio(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
 			return ES_DECODE_FAILED;
 	}
 
+	if (user_mode(ctxt->regs))
+		return ES_UNSUPPORTED;
+
 	switch (mmio) {
 	case INSN_MMIO_WRITE:
 		memcpy(ghcb->shared_buffer, reg_data, bytes);