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Commit aed5ed47 authored by Frederic Weisbecker's avatar Frederic Weisbecker Committed by Ingo Molnar
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context_tracking: Protect against recursion 



Context tracking recursion can happen when an exception triggers
in the middle of a call to a context tracking probe.

This special case can be caused by vmalloc faults. If an access
to a memory area allocated by vmalloc happens in the middle of
context_tracking_enter(), we may run into an endless fault loop
because the exception in turn calls context_tracking_enter()
which faults on the same vmalloc'ed memory, triggering an
exception again, etc...

Some rare crashes have been reported so lets protect against
this with a recursion counter.

Reported-by: default avatarDave Jones <davej@redhat.com>
Signed-off-by: default avatarFrederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: default avatarRik van Riel <riel@redhat.com>
Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Rafael J . Wysocki <rafael.j.wysocki@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1430928266-24888-2-git-send-email-fweisbec@gmail.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 0e1dc427

include/linux/context_tracking_state.h

+1 −0
Original line number Diff line number Diff line
@@ -12,6 +12,7 @@ struct context_tracking { 
	 * may be further optimized using static keys.

	 */

	bool active;

	int recursion;

	enum ctx_state {

		CONTEXT_KERNEL = 0,

		CONTEXT_USER,

kernel/context_tracking.c

+29 −0
Original line number Diff line number Diff line
@@ -38,6 +38,25 @@ void context_tracking_cpu_set(int cpu) 
	}

}



static bool context_tracking_recursion_enter(void)

{

	int recursion;



	recursion = __this_cpu_inc_return(context_tracking.recursion);

	if (recursion == 1)

		return true;



	WARN_ONCE((recursion < 1), "Invalid context tracking recursion value %d\n", recursion);

	__this_cpu_dec(context_tracking.recursion);



	return false;

}



static void context_tracking_recursion_exit(void)

{

	__this_cpu_dec(context_tracking.recursion);

}



/**

 * context_tracking_enter - Inform the context tracking that the CPU is going

 *                          enter user or guest space mode.
@@ -75,6 +94,9 @@ void context_tracking_enter(enum ctx_state state) 
	WARN_ON_ONCE(!current->mm);



	local_irq_save(flags);

	if (!context_tracking_recursion_enter())

		goto out_irq_restore;



	if ( __this_cpu_read(context_tracking.state) != state) {

		if (__this_cpu_read(context_tracking.active)) {

			/*
@@ -105,6 +127,8 @@ void context_tracking_enter(enum ctx_state state) 
		 */

		__this_cpu_write(context_tracking.state, state);

	}

	context_tracking_recursion_exit();

out_irq_restore:

	local_irq_restore(flags);

}

NOKPROBE_SYMBOL(context_tracking_enter);
@@ -139,6 +163,9 @@ void context_tracking_exit(enum ctx_state state) 
		return;



	local_irq_save(flags);

	if (!context_tracking_recursion_enter())

		goto out_irq_restore;



	if (__this_cpu_read(context_tracking.state) == state) {

		if (__this_cpu_read(context_tracking.active)) {

			/*
@@ -153,6 +180,8 @@ void context_tracking_exit(enum ctx_state state) 
		}

		__this_cpu_write(context_tracking.state, CONTEXT_KERNEL);

	}

	context_tracking_recursion_exit();

out_irq_restore:

	local_irq_restore(flags);

}

NOKPROBE_SYMBOL(context_tracking_exit);