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

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@suse.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jan Beulich <JBeulich@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/693c3bd7aeb4d3c27c92c622b7d0f554a458173c.1458162709.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/include/asm/xen/hypervisor.h b/arch/x86/include/asm/xen/hypervisor.h
index 8b2d4bea996273e1a72ee78aaa1a2b6358ccd189..39171b3646bba09d5c9a76e98632ae7339f6fbe8 100644 (file)
--- a/arch/x86/include/asm/xen/hypervisor.h
+++ b/arch/x86/include/asm/xen/hypervisor.h
@@ -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 */

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index b9d99e0f82c4f072a15f7cb3fb3434d391281daf..9f751876066f5c92e99adf7609a542cf9cc3550e 100644 (file)
--- 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);
 
@@ -411,6 +412,17 @@ __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(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

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 2c261082eadf82f693d062e7f600660fcd56817f..8381fb990c7faee0f0faa2aebddaf9fd3bfff6a2 100644 (file)
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -961,7 +961,7 @@ 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;