x86/entry/64: Prevent clobbering of saved CR2 value

The recent fix for CR2 corruption introduced a new way to reliably corrupt
the saved CR2 value.

CR2 is saved early in the entry code in RDX, which is the third argument to
the fault handling functions. But it missed that between saving and
invoking the fault handler enter_from_user_mode() can be called. RDX is a
caller saved register so the invoked function can freely clobber it with
the obvious consequences.

The TRACE_IRQS_OFF call is safe as it calls through the thunk which
preserves RDX, but TRACE_IRQS_OFF_DEBUG is not because it also calls into
C-code outside of the thunk.

Store CR2 in R12 instead which is a callee saved register and move R12 to
RDX just before calling the fault handler.

Fixes: a0d14b89 ("x86/mm, tracing: Fix CR2 corruption")
Reported-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/alpine.DEB.2.21.1907201020540.1782@nanos.tec.linutronix.de

arch/x86/entry/entry_64.S

@@ -875,7 +875,12 @@ apicinterrupt IRQ_WORK_VECTOR			irq_work_interrupt		smp_irq_work_interrupt
 	UNWIND_HINT_REGS
 
 	.if \read_cr2
-	GET_CR2_INTO(%rdx);			/* can clobber %rax */
+	/*
+	 * Store CR2 early so subsequent faults cannot clobber it. Use R12 as
+	 * intermediate storage as RDX can be clobbered in enter_from_user_mode().
+	 * GET_CR2_INTO can clobber RAX.
+	 */
+	GET_CR2_INTO(%r12);
 	.endif
 
 	.if \shift_ist != -1
@@ -904,6 +909,10 @@ apicinterrupt IRQ_WORK_VECTOR			irq_work_interrupt		smp_irq_work_interrupt
 	subq	$\ist_offset, CPU_TSS_IST(\shift_ist)
 	.endif
 
+	.if \read_cr2
+	movq	%r12, %rdx			/* Move CR2 into 3rd argument */
+	.endif
+
 	call	\do_sym
 
 	.if \shift_ist != -1