x86/iopl: Fix iopl capability check on Xen PV

commit c29016cf41fe9fa994a5ecca607cf5f1cd98801e upstream.

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>
Link: http://lkml.kernel.org/r/8ce12013e6e4c0a44a97e316be4a6faff31bd5ea.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/kernel/ioport.c b/arch/x86/kernel/ioport.c
index 37dae792dbbed00b480aa67a33cb0b2ae1699428..589b3193f1020b8389a5f64b296cff531e85e849 100644 (file)
--- 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;