[PATCH 11/11][CVE-2016-3157] x86/iopl/64: Properly context-switch IOPL on Xen PV

[Luis Henriques]

From: Andy Lutomirski <luto at kernel.org>

commit b7a584598aea7ca73140cb87b40319944dd3393f upstream.

On Xen PV, regs->flags doesn't reliably reflect IOPL and the
exit-to-userspace code doesn't change IOPL.  We need to context
switch it manually.

I'm doing this without going through paravirt because this is
specific to Xen PV.  After the dust settles, we can merge this with
the 32-bit code, tidy up the iopl syscall implementation, and remove
the set_iopl pvop entirely.

Fixes XSA-171.

Reviewewd-by: Jan Beulich <JBeulich at suse.com>
Signed-off-by: Andy Lutomirski <luto at kernel.org>
Cc: Andrew Cooper <andrew.cooper3 at citrix.com>
Cc: Andy Lutomirski <luto at amacapital.net>
Cc: Boris Ostrovsky <boris.ostrovsky at oracle.com>
Cc: Borislav Petkov <bp at alien8.de>
Cc: Brian Gerst <brgerst at gmail.com>
Cc: David Vrabel <david.vrabel at citrix.com>
Cc: Denys Vlasenko <dvlasenk at redhat.com>
Cc: H. Peter Anvin <hpa at zytor.com>
Cc: Jan Beulich <JBeulich at suse.com>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Thomas Gleixner <tglx at linutronix.de>
Link: http://lkml.kernel.org/r/693c3bd7aeb4d3c27c92c622b7d0f554a458173c.1458162709.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo at kernel.org>
[bwh: Backported to 3.2:
 - Use xen_pv_domain() directly as X86_FEATURE_XENPV is not defined
 - Adjust context]
Signed-off-by: Ben Hutchings <ben at decadent.org.uk>
CVE-2016-3157
BugLink: https://bugs.launchpad.net/bugs/1561388
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 arch/x86/include/asm/xen/hypervisor.h |  2 ++
 arch/x86/kernel/process_64.c          | 12 ++++++++++++
 arch/x86/xen/enlighten.c              |  2 +-
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/xen/hypervisor.h b/arch/x86/include/asm/xen/hypervisor.h
index 66d0fff1ee84..fc500f97b39d 100644
--- a/arch/x86/include/asm/xen/hypervisor.h
+++ b/arch/x86/include/asm/xen/hypervisor.h
@@ -72,4 +72,6 @@ static inline bool xen_x2apic_para_available(void)
 }
 #endif
 
+extern void xen_set_iopl_mask(unsigned mask);
+
 #endif /* _ASM_X86_XEN_HYPERVISOR_H */
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index ee2e70c9fdbf..557eb3757edb 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -38,6 +38,7 @@
 #include <linux/io.h>
 #include <linux/ftrace.h>
 #include <linux/cpuidle.h>
+#include <xen/xen.h>
 
 #include <asm/pgtable.h>
 #include <asm/system.h>
@@ -52,6 +53,7 @@
 #include <asm/syscalls.h>
 #include <asm/debugreg.h>
 #include <asm/nmi.h>
+#include <asm/xen/hypervisor.h>
 
 asmlinkage extern void ret_from_fork(void);
 
@@ -518,6 +520,16 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 		     task_thread_info(prev_p)->flags & _TIF_WORK_CTXSW_PREV))
 		__switch_to_xtra(prev_p, next_p, tss);
 
+#ifdef CONFIG_XEN
+	/*
+	 * On Xen PV, IOPL bits in pt_regs->flags have no effect, and
+	 * current_pt_regs()->flags may not match the current task's
+	 * intended IOPL.  We need to switch it manually.
+	 */
+	if (unlikely(xen_pv_domain() && prev->iopl != next->iopl))
+		xen_set_iopl_mask(next->iopl);
+#endif
+
 	return prev_p;
 }
 
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 9a331083a639..514d0d306972 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -790,7 +790,7 @@ static void xen_load_sp0(struct tss_struct *tss,
 	xen_mc_issue(PARAVIRT_LAZY_CPU);
 }
 
-static void xen_set_iopl_mask(unsigned mask)
+void xen_set_iopl_mask(unsigned mask)
 {
 	struct physdev_set_iopl set_iopl;