Patch "x86/iopl: Fix iopl capability check on Xen PV" has been added to the 4.5-stable tree

This is a note to let you know that I've just added the patch titled

x86/iopl: Fix iopl capability check on Xen PV

to the 4.5-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-iopl-fix-iopl-capability-check-on-xen-pv.patch
and it can be found in the queue-4.5 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Andy Lutomirski <[email protected]>
Date: Wed, 16 Mar 2016 14:14:22 -0700
Subject: x86/iopl: Fix iopl capability check on Xen PV

From: Andy Lutomirski <[email protected]>

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.

---
 arch/x86/kernel/ioport.c |   12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

@@ -96,9 +96,14 @@ asmlinkage long sys_ioperm(unsigned long
 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, leve
 		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;

Patches currently in stable-queue which might be from [email protected] are

queue-4.5/x86-iopl-64-properly-context-switch-iopl-on-xen-pv.patch queue-4.5/x86-iopl-fix-iopl-capability-check-on-xen-pv.patch queue-4.5/x86-entry-compat-keep-ts_compat-set-during-signal-delivery.patch