[RFC][PATCH 1/9] klp: Fix obvious RCU fail

[Peter Zijlstra]

While one must hold RCU-sched (aka. preempt_disable) for find_symbol()
one must equally hold it over the use of the object returned.

The moment you release the RCU-sched read lock, the object can be dead
and gone.

Cc: Seth Jennings <sjenning@xxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
Cc: Miroslav Benes <mbenes@xxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxx>
Cc: Jiri Kosina <jkosina@xxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/livepatch/core.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -248,11 +248,12 @@ static int klp_find_external_symbol(stru
 	/* first, check if it's an exported symbol */
 	preempt_disable();
 	sym = find_symbol(name, NULL, NULL, true, true);
-	preempt_enable();
 	if (sym) {
 		*addr = sym->value;
+		preempt_enable();
 		return 0;
 	}
+	preempt_enable();
 
 	/* otherwise check if it's in another .o within the patch module */
 	return klp_find_object_symbol(pmod->name, name, addr);


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