linux-kernel - [PATCH V2] x86/sev-es: Fix SEV-ES #VC handler for string port IO

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Date:   Sat,  9 Jan 2021 23:11:02 -0800
From:   "Hyunwook (Wooky) Baek" <baekhw@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
Cc:     Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        David Rientjes <rientjes@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        "Hyunwook (Wooky) Baek" <baekhw@...gle.com>
Subject: [PATCH V2] x86/sev-es: Fix SEV-ES #VC handler for string port IO

Don't assume dest/source buffers are userspace addresses when manually
copying data for string I/O or MOVS MMIO, as {get,put}_user() will fail
if handed a kernel address and ultimately lead to a kernel panic.

Signed-off-by: Hyunwook (Wooky) Baek <baekhw@...gle.com>
Acked-by: David Rientjes <rientjes@...gle.com>
---

This patch is tested by invoking INSB/OUTSB instructions in kernel space in a
SEV-ES-enabled VM. Without the patch, the kernel crashed with the following
message:
  "SEV-ES: Unsupported exception in #VC instruction emulation - can't continue"
With the patch, the instructions successfully read/wrote the string from/to
the I/O port.

 arch/x86/kernel/sev-es.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
index 0bd1a0fc587e..ab31c34ba508 100644
--- a/arch/x86/kernel/sev-es.c
+++ b/arch/x86/kernel/sev-es.c
@@ -286,6 +286,12 @@ static enum es_result vc_write_mem(struct es_em_ctxt *ctxt,
 	u16 d2;
 	u8  d1;

+	/* If instruction ran in kernel mode and the I/O buffer is in kernel space */
+	if (!user_mode(ctxt->regs) && !access_ok(target, size)) {
+		memcpy(dst, buf, size);
+		return ES_OK;
+	}
+
 	switch (size) {
 	case 1:
 		memcpy(&d1, buf, 1);
@@ -335,6 +341,12 @@ static enum es_result vc_read_mem(struct es_em_ctxt *ctxt,
 	u16 d2;
 	u8  d1;

+	/* If instruction ran in kernel mode and the I/O buffer is in kernel space */
+	if (!user_mode(ctxt->regs) && !access_ok(s, size)) {
+		memcpy(buf, src, size);
+		return ES_OK;
+	}
+
 	switch (size) {
 	case 1:
 		if (get_user(d1, s))
--
2.30.0.284.gd98b1dd5eaa7-goog