[PATCH v4 3/3] x86/iopl: Fix iopl capability check on Xen PV

[Andy Lutomirski]

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

Cc: sta...@vger.kernel.org
Reviewewd-by: Jan Beulich <jbeul...@suse.com>
Signed-off-by: Andy Lutomirski <l...@kernel.org>
---
 arch/x86/kernel/ioport.c | 12 +++++++++---
 1 file 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;
-- 
2.5.0