linux-kernel - [patch V4 part 4 15/24] x86/db: Split out dr6/7 handling

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Date:   Tue, 05 May 2020 15:49:41 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, "Paul E. McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: [patch V4 part 4 15/24] x86/db: Split out dr6/7 handling

From: Peter Zijlstra <peterz@...radead.org>

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@...radead.org>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 arch/x86/kernel/hw_breakpoint.c |    6 ---
 arch/x86/kernel/traps.c         |   62 +++++++++++++++++++++++++++-------------
 2 files changed, 44 insertions(+), 24 deletions(-)

--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -464,7 +464,7 @@ static int hw_breakpoint_handler(struct
 {
 	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
 	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
 	    (dr6 & (~DR_TRAP_BITS)))
 		rc = NOTIFY_DONE;
 
-	set_debugreg(dr7, 7);
 	put_cpu();
 
 	return rc;
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -691,6 +691,44 @@ static bool is_sysenter_singlestep(struc
 #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, 6);
+	set_debugreg(0, 7);
+
+	/*
+	 * 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)
+{
+	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
@@ -718,28 +756,13 @@ static bool is_sysenter_singlestep(struc
 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;
 
-	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);
+	debug_enter(&dr6, &dr7);
 
-	/* Filter out all the reserved bits which are preset to 1 */
-	dr6 &= ~DR6_RESERVED;
+	nmi_enter();
 
 	/*
 	 * The SDM says "The processor clears the BTF flag when it
@@ -777,7 +800,7 @@ dotraplinkage void do_debug(struct pt_re
 #endif
 
 	if (notify_die(DIE_DEBUG, "debug", regs, (long)&dr6, error_code,
-							SIGTRAP) == NOTIFY_STOP)
+		       SIGTRAP) == NOTIFY_STOP)
 		goto exit;
 
 	/*
@@ -816,6 +839,7 @@ dotraplinkage void do_debug(struct pt_re
 
 exit:
 	nmi_exit();
+	debug_exit(dr7);
 }
 NOKPROBE_SYMBOL(do_debug);