Patch "x86/iopl/64: Properly context-switch IOPL on Xen PV" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-iopl-64-properly-context-switch-iopl-on-xen-pv.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Andy Lutomirski <[email protected]>
Date: Wed, 16 Mar 2016 14:14:21 -0700
Subject: x86/iopl/64: Properly context-switch IOPL on Xen PV

From: Andy Lutomirski <[email protected]>

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.

---
 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(-)

@@ -62,4 +62,6 @@ void xen_arch_register_cpu(int num);
 void xen_arch_unregister_cpu(int num);
 #endif
 
+extern void xen_set_iopl_mask(unsigned mask);
+
 #endif /* _ASM_X86_XEN_HYPERVISOR_H */

@@ -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);
 

@@ -411,6 +412,17 @@ __switch_to(struct task_struct *prev_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(static_cpu_has(X86_FEATURE_XENPV) &&
+		     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

@@ -961,7 +961,7 @@ static void xen_load_sp0(struct tss_stru
 	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;
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/x86-iopl-64-properly-context-switch-iopl-on-xen-pv.patch queue-4.4/x86-iopl-fix-iopl-capability-check-on-xen-pv.patch queue-4.4/x86-entry-compat-keep-ts_compat-set-during-signal-delivery.patch