linux-kernel - [PATCH 4.4 34/37] kaiser: disabled on Xen PV

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Date:   Wed,  3 Jan 2018 21:11:40 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Jiri Kosina <jkosina@...e.cz>
Subject: [PATCH 4.4 34/37] kaiser: disabled on Xen PV

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jiri Kosina <jkosina@...e.cz>


Kaiser cannot be used on paravirtualized MMUs (namely reading and writing CR3).
This does not work with KAISER as the CR3 switch from and to user space PGD
would require to map the whole XEN_PV machinery into both.

More importantly, enabling KAISER on Xen PV doesn't make too much sense, as PV
guests use distinct %cr3 values for kernel and user already.

Signed-off-by: Jiri Kosina <jkosina@...e.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/x86/mm/kaiser.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/arch/x86/mm/kaiser.c
+++ b/arch/x86/mm/kaiser.c
@@ -264,6 +264,9 @@ void __init kaiser_check_boottime_disabl
 	char arg[5];
 	int ret;
 
+	if (boot_cpu_has(X86_FEATURE_XENPV))
+		goto silent_disable;
+
 	ret = cmdline_find_option(boot_command_line, "pti", arg, sizeof(arg));
 	if (ret > 0) {
 		if (!strncmp(arg, "on", 2))
@@ -291,6 +294,8 @@ enable:
 
 disable:
 	pr_info("Kernel/User page tables isolation: disabled\n");
+
+silent_disable:
 	kaiser_enabled = 0;
 	setup_clear_cpu_cap(X86_FEATURE_KAISER);
 }