linux-kernel - [PATCH] xen/arm: do not setup the runstate info page if kpti is enabled

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Date:   Thu, 24 Sep 2020 16:49:55 -0700
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     sstabellini@...nel.org,
        Stefano Stabellini <stefano.stabellini@...inx.com>,
        Bertrand Marquis <Bertrand.Marquis@....com>,
        boris.ostrovsky@...cle.com, jgross@...e.com
Subject: [PATCH] xen/arm: do not setup the runstate info page if kpti is enabled

From: Stefano Stabellini <stefano.stabellini@...inx.com>

The VCPUOP_register_runstate_memory_area hypercall takes a virtual
address of a buffer as a parameter. The semantics of the hypercall are
such that the virtual address should always be valid.

When KPTI is enabled and we are running userspace code, the virtual
address is not valid, thus, Linux is violating the semantics of
VCPUOP_register_runstate_memory_area.

Do not call VCPUOP_register_runstate_memory_area when KPTI is enabled.

Signed-off-by: Stefano Stabellini <stefano.stabellini@...inx.com>
CC: Bertrand Marquis <Bertrand.Marquis@....com>
CC: boris.ostrovsky@...cle.com
CC: jgross@...e.com
---
 arch/arm/include/asm/xen/page.h   | 5 +++++
 arch/arm/xen/enlighten.c          | 6 ++++--
 arch/arm64/include/asm/xen/page.h | 6 ++++++
 3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/arch/arm/include/asm/xen/page.h b/arch/arm/include/asm/xen/page.h
index 31bbc803cecb..dc7f6e91aafa 100644
--- a/arch/arm/include/asm/xen/page.h
+++ b/arch/arm/include/asm/xen/page.h
@@ -1 +1,6 @@
 #include <xen/arm/page.h>
+
+static inline bool xen_kernel_unmapped_at_usr(void)
+{
+	return false;
+}
diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
index e93145d72c26..ea76562af1e9 100644
--- a/arch/arm/xen/enlighten.c
+++ b/arch/arm/xen/enlighten.c
@@ -158,7 +158,8 @@ static int xen_starting_cpu(unsigned int cpu)
 	BUG_ON(err);
 	per_cpu(xen_vcpu, cpu) = vcpup;
 
-	xen_setup_runstate_info(cpu);
+	if (!xen_kernel_unmapped_at_usr())
+		xen_setup_runstate_info(cpu);
 
 after_register_vcpu_info:
 	enable_percpu_irq(xen_events_irq, 0);
@@ -387,7 +388,8 @@ static int __init xen_guest_init(void)
 		return -EINVAL;
 	}
 
-	xen_time_setup_guest();
+	if (!xen_kernel_unmapped_at_usr())
+		xen_time_setup_guest();
 
 	if (xen_initial_domain())
 		pvclock_gtod_register_notifier(&xen_pvclock_gtod_notifier);
diff --git a/arch/arm64/include/asm/xen/page.h b/arch/arm64/include/asm/xen/page.h
index 31bbc803cecb..dffdc773221b 100644
--- a/arch/arm64/include/asm/xen/page.h
+++ b/arch/arm64/include/asm/xen/page.h
@@ -1 +1,7 @@
 #include <xen/arm/page.h>
+#include <asm/mmu.h>
+
+static inline bool xen_kernel_unmapped_at_usr(void)
+{
+	return arm64_kernel_unmapped_at_el0();
+}
-- 
2.17.1