x86/iopl: Fix iopl capability check on Xen PV

iopl(3) is supposed to work if iopl is already 3, even if
unprivileged.  This didn't work right on Xen PV.  Fix it.

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/8ce12013e6e4c0a44a97e316be4a6faff31bd5ea.1458162709.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 9 insertions, 3 deletions

diff --git a/arch/x86/kernel/ioport.c b/arch/x86/kernel/ioport.c
index 37dae792dbbe..589b3193f102 100644
--- a/arch/x86/kernel/ioport.c
+++ b/arch/x86/kernel/ioport.c
@@ -96,9 +96,14 @@ asmlinkage long sys_ioperm(unsigned long from, unsigned long num, int turn_on)
 SYSCALL_DEFINE1(iopl, unsigned int, level)
 {
         struct pt_regs *regs = current_pt_regs();
-        unsigned int old = (regs->flags >> 12) & 3;
         struct thread_struct *t = &current->thread;
 
+        /*
+         * Careful: the IOPL bits in regs->flags are undefined under Xen PV
+         * and changing them has no effect.
+         */
+        unsigned int old = t->iopl >> X86_EFLAGS_IOPL_BIT;
+
         if (level > 3)
                 return -EINVAL;
         /* Trying to gain more privileges? */
@@ -106,8 +111,9 @@ SYSCALL_DEFINE1(iopl, unsigned int, level)
                 if (!capable(CAP_SYS_RAWIO))
                         return -EPERM;
         }
-        regs->flags = (regs->flags & ~X86_EFLAGS_IOPL) | (level << 12);
-        t->iopl = level << 12;
+        regs->flags = (regs->flags & ~X86_EFLAGS_IOPL) |
+                (level << X86_EFLAGS_IOPL_BIT);
+        t->iopl = level << X86_EFLAGS_IOPL_BIT;
         set_iopl_mask(t->iopl);
 
         return 0;