x86/dumpstack: Pin the target stack when dumping it

Specifically, pin the stack in save_stack_trace_tsk() and
show_trace_log_lvl().

This will prevent a crash if the target task dies before or while
dumping its stack once we start freeing task stacks early.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
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: Jann Horn <jann@thejh.net>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/cf0082cde65d1941a996d026f2b2cdbfaca17bfa.1474003868.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/dumpstack_32.c

@@ -163,6 +163,9 @@ show_stack_log_lvl(struct task_struct *task, struct pt_regs *regs,
 	unsigned long *stack;
 	int i;
 
+	if (!try_get_task_stack(task))
+		return;
+
 	sp = sp ? : get_stack_pointer(task, regs);
 
 	stack = sp;
@@ -179,6 +182,8 @@ show_stack_log_lvl(struct task_struct *task, struct pt_regs *regs,
 	}
 	pr_cont("\n");
 	show_trace_log_lvl(task, regs, sp, bp, log_lvl);
+
+	put_task_stack(task);
 }
 
 

arch/x86/kernel/dumpstack_64.c

@@ -218,6 +218,9 @@ show_stack_log_lvl(struct task_struct *task, struct pt_regs *regs,
 	unsigned long *stack;
 	int i;
 
+	if (!try_get_task_stack(task))
+		return;
+
 	irq_stack_end = (unsigned long *)this_cpu_read(irq_stack_ptr);
 	irq_stack     = irq_stack_end - (IRQ_STACK_SIZE / sizeof(long));
 
@@ -253,6 +256,8 @@ show_stack_log_lvl(struct task_struct *task, struct pt_regs *regs,
 
 	pr_cont("\n");
 	show_trace_log_lvl(task, regs, sp, bp, log_lvl);
+
+	put_task_stack(task);
 }
 
 void show_regs(struct pt_regs *regs)

arch/x86/kernel/stacktrace.c

@@ -79,9 +79,14 @@ void save_stack_trace_regs(struct pt_regs *regs, struct stack_trace *trace)
 
 void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
 {
+	if (!try_get_task_stack(tsk))
+		return;
+
 	dump_trace(tsk, NULL, NULL, 0, &save_stack_ops_nosched, trace);
 	if (trace->nr_entries < trace->max_entries)
 		trace->entries[trace->nr_entries++] = ULONG_MAX;
+
+	put_task_stack(tsk);
 }
 EXPORT_SYMBOL_GPL(save_stack_trace_tsk);