From a37cd2a59d0cb270b1bba568fd3a3b8668b9d3ba Mon Sep 17 00:00:00 2001
From: "Borislav Petkov (AMD)" <bp@alien8.de>
Date: Thu, 5 Oct 2023 11:06:36 +0200
Subject: [PATCH] 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: 0118b604c2c9 ("x86/sev-es: Handle MMIO String Instructions")
Reported-by: Tom Dohrmann <erbse.13@gmx.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Tom Dohrmann <erbse.13@gmx.de>
Cc: <stable@kernel.org>
---
arch/x86/kernel/sev.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 2787826d9f607..4ee14e647ae62 100644
--- 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);
--
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