x86/entry/64: Fix an IRQ state error on ptregs-using syscalls

I messed up the IRQ state when jumping off the fast path due to
invocation of a ptregs-using syscall.  This bug shouldn't have
had any impact yet, but it would have caused problems with
subsequent context tracking cleanups.
Reported-and-tested-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 1e423bff x86/entry/64: ("Migrate the 64-bit syscall slow path to C")
Link: http://lkml.kernel.org/r/ab92cd365fb7b0a56869e920017790d96610fdca.1454261517.git.luto@kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/entry_64.S

@@ -191,8 +191,8 @@ entry_SYSCALL_64_fastpath:
 
 	/*
 	 * This call instruction is handled specially in stub_ptregs_64.
-	 * It might end up jumping to the slow path.  If it jumps, RAX is
-	 * clobbered.
+	 * It might end up jumping to the slow path.  If it jumps, RAX
+	 * and all argument registers are clobbered.
 	 */
 	call	*sys_call_table(, %rax, 8)
 .Lentry_SYSCALL_64_after_fastpath_call:
@@ -315,17 +315,24 @@ END(entry_SYSCALL_64)
 ENTRY(stub_ptregs_64)
 	/*
 	 * Syscalls marked as needing ptregs land here.
-	 * If we are on the fast path, we need to save the extra regs.
-	 * If we are on the slow path, the extra regs are already saved.
+	 * If we are on the fast path, we need to save the extra regs,
+	 * which we achieve by trying again on the slow path.  If we are on
+	 * the slow path, the extra regs are already saved.
 	 *
 	 * RAX stores a pointer to the C function implementing the syscall.
+	 * IRQs are on.
 	 */
 	cmpq	$.Lentry_SYSCALL_64_after_fastpath_call, (%rsp)
 	jne	1f
 
-	/* Called from fast path -- pop return address and jump to slow path */
+	/*
+	 * Called from fast path -- disable IRQs again, pop return address
+	 * and jump to slow path
+	 */
+	DISABLE_INTERRUPTS(CLBR_NONE)
+	TRACE_IRQS_OFF
 	popq	%rax
-	jmp	entry_SYSCALL64_slow_path	/* called from fast path */
+	jmp	entry_SYSCALL64_slow_path
 
 1:
 	/* Called from C */