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@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>
Link: http://lkml.kernel.org/r/693c3bd7aeb4d3c27c92c622b7d0f554a458173c.1458162709.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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 e835d263a33b43ccf7601698cfeadc447ede491e..4cbb60fbff3e361f3e1931a4ab6dae26363db871 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 b7de78bdc09c12b3e6cea070e4d35b346d24c8be..beab8c706ac95070f02b683df74b056f0d53ca64 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;