linux-kernel - [PATCH v2 5/5] x86/sev-es: Do not support MMIO to/from encrypted memory

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Date:   Tue, 20 Oct 2020 14:18:56 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     x86@...nel.org
Cc:     Joerg Roedel <joro@...tes.org>, Joerg Roedel <jroedel@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Martin Radev <martin.b.radev@...il.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2 5/5] x86/sev-es: Do not support MMIO to/from encrypted memory

From: Joerg Roedel <jroedel@...e.de>

MMIO memory is usually not mapped encrypted, so there is no reason to
support emulated MMIO when it is mapped encrypted.

This prevents a possible hypervisor attack where it maps a RAM page as
an MMIO page in the nested page-table, so that any guest access to it
will trigger a #VC exception and leak the data on that page to the
hypervisor or allows the hypervisor to inject data into the guest.

Signed-off-by: Joerg Roedel <jroedel@...e.de>
---
 arch/x86/kernel/sev-es.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
index 4a96726fbaf8..421fe0203c68 100644
--- a/arch/x86/kernel/sev-es.c
+++ b/arch/x86/kernel/sev-es.c
@@ -374,8 +374,8 @@ static enum es_result vc_read_mem(struct es_em_ctxt *ctxt,
 	return ES_EXCEPTION;
 }
 
-static bool vc_slow_virt_to_phys(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
-				 unsigned long vaddr, phys_addr_t *paddr)
+static enum es_result vc_slow_virt_to_phys(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
+					   unsigned long vaddr, phys_addr_t *paddr)
 {
 	unsigned long va = (unsigned long)vaddr;
 	unsigned int level;
@@ -394,15 +394,19 @@ static bool vc_slow_virt_to_phys(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
 		if (user_mode(ctxt->regs))
 			ctxt->fi.error_code |= X86_PF_USER;
 
-		return false;
+		return ES_EXCEPTION;
 	}
 
+	if (unlikely(WARN_ON_ONCE(pte_val(*pte) & _PAGE_ENC)))
+		/* Emulated MMIO to/from encrypted memory not supported */
+		return ES_UNSUPPORTED;
+
 	pa = (phys_addr_t)pte_pfn(*pte) << PAGE_SHIFT;
 	pa |= va & ~page_level_mask(level);
 
 	*paddr = pa;
 
-	return true;
+	return ES_OK;
 }
 
 /* Include code shared with pre-decompression boot stage */
@@ -731,6 +735,7 @@ static enum es_result vc_do_mmio(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
 {
 	u64 exit_code, exit_info_1, exit_info_2;
 	unsigned long ghcb_pa = __pa(ghcb);
+	enum es_result res;
 	phys_addr_t paddr;
 	void __user *ref;
 
@@ -740,11 +745,12 @@ static enum es_result vc_do_mmio(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
 
 	exit_code = read ? SVM_VMGEXIT_MMIO_READ : SVM_VMGEXIT_MMIO_WRITE;
 
-	if (!vc_slow_virt_to_phys(ghcb, ctxt, (unsigned long)ref, &paddr)) {
-		if (!read)
+	res = vc_slow_virt_to_phys(ghcb, ctxt, (unsigned long)ref, &paddr);
+	if (res != ES_OK) {
+		if (res == ES_EXCEPTION && !read)
 			ctxt->fi.error_code |= X86_PF_WRITE;
 
-		return ES_EXCEPTION;
+		return res;
 	}
 
 	exit_info_1 = paddr;
-- 
2.28.0