x86/db: Split out dr6/7 handling

DR6/7 should be handled before nmi_enter() is invoked and restore after
nmi_exit() to minimize the exposure.

Split it out into helper inlines and bring it into the correct order.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20200505135314.808628211@linutronix.de

arch/x86/kernel/hw_breakpoint.c

@@ -464,7 +464,7 @@ static int hw_breakpoint_handler(struct die_args *args)
 {
 	int i, cpu, rc = NOTIFY_STOP;
 	struct perf_event *bp;
-	unsigned long dr7, dr6;
+	unsigned long dr6;
 	unsigned long *dr6_p;
 
 	/* The DR6 value is pointed by args->err */
@@ -479,9 +479,6 @@ static int hw_breakpoint_handler(struct die_args *args)
 	if ((dr6 & DR_TRAP_BITS) == 0)
 		return NOTIFY_DONE;
 
-	get_debugreg(dr7, 7);
-	/* Disable breakpoints during exception handling */
-	set_debugreg(0UL, 7);
 	/*
 	 * Assert that local interrupts are disabled
 	 * Reset the DRn bits in the virtualized register value.
@@ -538,7 +535,6 @@ static int hw_breakpoint_handler(struct die_args *args)
 	    (dr6 & (~DR_TRAP_BITS)))
 		rc = NOTIFY_DONE;
 
-	set_debugreg(dr7, 7);
 	put_cpu();
 
 	return rc;

arch/x86/kernel/traps.c

@@ -700,6 +700,57 @@ static bool is_sysenter_singlestep(struct pt_regs *regs)
 #endif
 }
 
+static __always_inline void debug_enter(unsigned long *dr6, unsigned long *dr7)
+{
+	/*
+	 * Disable breakpoints during exception handling; recursive exceptions
+	 * are exceedingly 'fun'.
+	 *
+	 * Since this function is NOKPROBE, and that also applies to
+	 * HW_BREAKPOINT_X, we can't hit a breakpoint before this (XXX except a
+	 * HW_BREAKPOINT_W on our stack)
+	 *
+	 * Entry text is excluded for HW_BP_X and cpu_entry_area, which
+	 * includes the entry stack is excluded for everything.
+	 */
+	get_debugreg(*dr7, 7);
+	set_debugreg(0, 7);
+
+	/*
+	 * Ensure the compiler doesn't lower the above statements into
+	 * the critical section; disabling breakpoints late would not
+	 * be good.
+	 */
+	barrier();
+
+	/*
+	 * The Intel SDM says:
+	 *
+	 *   Certain debug exceptions may clear bits 0-3. The remaining
+	 *   contents of the DR6 register are never cleared by the
+	 *   processor. To avoid confusion in identifying debug
+	 *   exceptions, debug handlers should clear the register before
+	 *   returning to the interrupted task.
+	 *
+	 * Keep it simple: clear DR6 immediately.
+	 */
+	get_debugreg(*dr6, 6);
+	set_debugreg(0, 6);
+	/* Filter out all the reserved bits which are preset to 1 */
+	*dr6 &= ~DR6_RESERVED;
+}
+
+static __always_inline void debug_exit(unsigned long dr7)
+{
+	/*
+	 * Ensure the compiler doesn't raise this statement into
+	 * the critical section; enabling breakpoints early would
+	 * not be good.
+	 */
+	barrier();
+	set_debugreg(dr7, 7);
+}
+
 /*
  * Our handling of the processor debug registers is non-trivial.
  * We do not clear them on entry and exit from the kernel. Therefore
@@ -727,29 +778,14 @@ static bool is_sysenter_singlestep(struct pt_regs *regs)
 dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
 {
 	struct task_struct *tsk = current;
+	unsigned long dr6, dr7;
 	int user_icebp = 0;
-	unsigned long dr6;
 	int si_code;
 
+	debug_enter(&dr6, &dr7);
+
 	nmi_enter();
 
-	get_debugreg(dr6, 6);
-	/*
-	 * The Intel SDM says:
-	 *
-	 *   Certain debug exceptions may clear bits 0-3. The remaining
-	 *   contents of the DR6 register are never cleared by the
-	 *   processor. To avoid confusion in identifying debug
-	 *   exceptions, debug handlers should clear the register before
-	 *   returning to the interrupted task.
-	 *
-	 * Keep it simple: clear DR6 immediately.
-	 */
-	set_debugreg(0, 6);
-
-	/* Filter out all the reserved bits which are preset to 1 */
-	dr6 &= ~DR6_RESERVED;
-
 	/*
 	 * The SDM says "The processor clears the BTF flag when it
 	 * generates a debug exception."  Clear TIF_BLOCKSTEP to keep
@@ -786,7 +822,7 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
 #endif
 
 	if (notify_die(DIE_DEBUG, "debug", regs, (long)&dr6, error_code,
-							SIGTRAP) == NOTIFY_STOP)
+		       SIGTRAP) == NOTIFY_STOP)
 		goto exit;
 
 	/*
@@ -825,6 +861,7 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
 
 exit:
 	nmi_exit();
+	debug_exit(dr7);
 }
 NOKPROBE_SYMBOL(do_debug);