Patch "x86/ioapic: Prevent NULL pointer dereference in setup_ioapic_dest()" has been added to the 4.2-stable tree

This is a note to let you know that I've just added the patch titled

x86/ioapic: Prevent NULL pointer dereference in setup_ioapic_dest()

to the 4.2-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-ioapic-prevent-null-pointer-dereference-in-setup_ioapic_dest.patch
and it can be found in the queue-4.2 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Werner Pawlitschko <[email protected]>
Date: Tue, 27 Oct 2015 09:08:04 +0900
Subject: x86/ioapic: Prevent NULL pointer dereference in setup_ioapic_dest()

From: Werner Pawlitschko <[email protected]>

commit ababae44108b0e94b58eef6cb5bd830bd040a47f upstream.

Commit 4857c91f0d19 changed the way how irq affinity is setup in setup_ioapic_dest() from using the core helper function to unconditionally calling the irq_set_affinity() callback of the underlying irq chip.

That results in a NULL pointer dereference for the rare case where the underlying irq chip is lapic_chip which has no irq_set_affinity() callback. lapic_chip is occasionally used for the timer interrupt (irq 0).

The fix is simple: Check the availability of the callback instead of calling it unconditionally.

---
 arch/x86/kernel/apic/io_apic.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

@@ -2547,7 +2547,9 @@ void __init setup_ioapic_dest(void)
 			mask = apic->target_cpus();
 
 		chip = irq_data_get_irq_chip(idata);
-		chip->irq_set_affinity(idata, mask, false);
+		/* Might be lapic_chip for irq 0 */
+		if (chip->irq_set_affinity)
+			chip->irq_set_affinity(idata, mask, false);
 	}
 }
 #endif

Patches currently in stable-queue which might be from [email protected] are

queue-4.2/x86-ioapic-prevent-null-pointer-dereference-in-setup_ioapic_dest.patch