[PATCH] Fix kallsyms/insmod/rmmod race

The attached patch fixes a race between kallsyms and insmod/rmmod.

The problem is this:

 (1) The various kallsyms functions poke around in the module list without any
     locking so that they can be called from the oops handler.

 (2) Although insmod and rmmod use locks to exclude each other, these have no
     effect on the kallsyms function.

 (3) Although rmmod modifies the module state with the machine "stopped", it
     hasn't removed the metadata from the module metadata list, meaning that
     as soon as the machine is "restarted", the metadata can be observed by
     kallsyms.

     It's not possible to say that an item in that list should be ignored if
     it's state is marked as inactive - you can't get at the state information
     because you can't trust the metadata in which it is embedded.

     Furthermore, list linkage information is embedded in the metadata too, so
     you can't trust that either...

 (4) kallsyms may be walking the module list without a lock whilst either
     insmod or rmmod are busy changing it. insmod probably isn't a problem
     since nothing is going a way, but rmmod is as it's deleting an entry.

 (5) Therefore nothing that uses these functions can in any way trust any
     pointers to "static" data (such as module symbol names or module names)
     that are returned.

 (6) On ppc64 the problems are exacerbated since the hypervisor may reschedule
     bits of the kernel, making operations that appear adjacent occur a long
     time apart.

This patch fixes the race by only linking/unlinking modules into/from the
master module list with the machine in the "stopped" state. This means that
any "static" information can be trusted as far as the next kernel reschedule
on any given CPU without the need to hold any locks.

However, I'm not sure how this is affected by preemption. I suspect more work
may need to be done in that case, but I'm not entirely sure.

This also means that rmmod has to bump the machine into the stopped state
twice... but since that shouldn't be a common operation, I don't think that's
a problem.

I've amended this patch to not get spinlocks whilst in the machine locked
state - there's no point as nothing else can be holding spinlocks.

Signed-Off-By: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/include/linux/stop_machine.h b/include/linux/stop_machine.h
index 6f43cb53f21b..151a803ed0ed 100644
--- a/include/linux/stop_machine.h
+++ b/include/linux/stop_machine.h
@@ -8,7 +8,7 @@
 #include <linux/cpu.h>
 #include <asm/system.h>
 
-#ifdef CONFIG_SMP
+#if defined(CONFIG_STOP_MACHINE) && defined(CONFIG_SMP)
 /**
  * stop_machine_run: freeze the machine on all CPUs and run this function
  * @fn: the function to run