[4.2.y-ckt stable] Patch "x86/iopl/64: Properly context-switch IOPL on Xen PV" has been added to the 4.2.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    x86/iopl/64: Properly context-switch IOPL on Xen PV

to the linux-4.2.y-queue branch of the 4.2.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-4.2.y-queue

This patch is scheduled to be released in version 4.2.8-ckt7.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 4.2.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From 440b7e60f79c3673b0112882db09a2ce73b08035 Mon Sep 17 00:00:00 2001
From: Andy Lutomirski <luto at kernel.org>
Date: Wed, 16 Mar 2016 14:14:21 -0700
Subject: x86/iopl/64: Properly context-switch IOPL on Xen PV

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>
[ kamal: backport to 4.2-stable: no X86_FEATURE_XENPV so just call
  xen_pv_domain() directly ]
Acked-by: Andy Lutomirski <luto at kernel.org>
Signed-off-by: Kamal Mostafa <kamal 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 d866959..d2ad00a 100644
--- a/arch/x86/include/asm/xen/hypervisor.h
+++ b/arch/x86/include/asm/xen/hypervisor.h
@@ -57,4 +57,6 @@ 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 adff851..87a848b 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -48,6 +48,7 @@
 #include <asm/syscalls.h>
 #include <asm/debugreg.h>
 #include <asm/switch_to.h>
+#include <asm/xen/hypervisor.h>

 asmlinkage extern void ret_from_fork(void);

@@ -417,6 +418,17 @@ __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
+
 	if (static_cpu_has_bug(X86_BUG_SYSRET_SS_ATTRS)) {
 		/*
 		 * AMD CPUs have a misfeature: SYSRET sets the SS selector but
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index f83c0ba..79858f7 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -959,7 +959,7 @@ static void xen_load_sp0(struct tss_struct *tss,
 	tss->x86_tss.sp0 = thread->sp0;
 }

-static void xen_set_iopl_mask(unsigned mask)
+void xen_set_iopl_mask(unsigned mask)
 {
 	struct physdev_set_iopl set_iopl;

--
2.7.4