patches/fix-rt-int3-x86_32-3.2-rt.patch

From: Steven Rostedt <rostedt@goodmis.org>
Date: Tue, 14 Jul 2015 14:26:34 +0200
Subject: x86: Do not disable preemption in int3 on 32bit

Preemption must be disabled before enabling interrupts in do_trap
on x86_64 because the stack in use for int3 and debug is a per CPU
stack set by th IST. But 32bit does not have an IST and the stack
still belongs to the current task and there is no problem in scheduling
out the task.

Keep preemption enabled on X86_32 when enabling interrupts for
do_trap().

The name of the function is changed from preempt_conditional_sti/cli()
to conditional_sti/cli_ist(), to annotate that this function is used
when the stack is on the IST.


Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

---
 arch/x86/kernel/traps.c |   28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -88,9 +88,21 @@ static inline void conditional_sti(struc
 		local_irq_enable();
 }
 
-static inline void preempt_conditional_sti(struct pt_regs *regs)
+static inline void conditional_sti_ist(struct pt_regs *regs)
 {
+#ifdef CONFIG_X86_64
+	/*
+	 * X86_64 uses a per CPU stack on the IST for certain traps
+	 * like int3. The task can not be preempted when using one
+	 * of these stacks, thus preemption must be disabled, otherwise
+	 * the stack can be corrupted if the task is scheduled out,
+	 * and another task comes in and uses this stack.
+	 *
+	 * On x86_32 the task keeps its own stack and it is OK if the
+	 * task schedules out.
+	 */
 	preempt_count_inc();
+#endif
 	if (regs->flags & X86_EFLAGS_IF)
 		local_irq_enable();
 }
@@ -101,11 +113,13 @@ static inline void conditional_cli(struc
 		local_irq_disable();
 }
 
-static inline void preempt_conditional_cli(struct pt_regs *regs)
+static inline void conditional_cli_ist(struct pt_regs *regs)
 {
 	if (regs->flags & X86_EFLAGS_IF)
 		local_irq_disable();
+#ifdef CONFIG_X86_64
 	preempt_count_dec();
+#endif
 }
 
 enum ctx_state ist_enter(struct pt_regs *regs)
@@ -536,9 +550,9 @@ dotraplinkage void notrace do_int3(struc
 	 * as we may switch to the interrupt stack.
 	 */
 	debug_stack_usage_inc();
-	preempt_conditional_sti(regs);
+	conditional_sti_ist(regs);
 	do_trap(X86_TRAP_BP, SIGTRAP, "int3", regs, error_code, NULL);
-	preempt_conditional_cli(regs);
+	conditional_cli_ist(regs);
 	debug_stack_usage_dec();
 exit:
 	ist_exit(regs, prev_state);
@@ -668,12 +682,12 @@ dotraplinkage void do_debug(struct pt_re
 	debug_stack_usage_inc();
 
 	/* It's safe to allow irq's after DR6 has been saved */
-	preempt_conditional_sti(regs);
+	conditional_sti_ist(regs);
 
 	if (v8086_mode(regs)) {
 		handle_vm86_trap((struct kernel_vm86_regs *) regs, error_code,
 					X86_TRAP_DB);
-		preempt_conditional_cli(regs);
+		conditional_cli_ist(regs);
 		debug_stack_usage_dec();
 		goto exit;
 	}
@@ -693,7 +707,7 @@ dotraplinkage void do_debug(struct pt_re
 	si_code = get_si_code(tsk->thread.debugreg6);
 	if (tsk->thread.debugreg6 & (DR_STEP | DR_TRAP_BITS) || user_icebp)
 		send_sigtrap(tsk, regs, error_code, si_code);
-	preempt_conditional_cli(regs);
+	conditional_cli_ist(regs);
 	debug_stack_usage_dec();
 
 exit: