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authorPeter Zijlstra <peterz@infradead.org>2015-08-20 10:34:59 +0930
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2015-11-09 12:50:25 -0800
commit9f0a848f82684f8f1417b3770a679a4441486bf7 (patch)
tree20cec9ced7bf66002bde6b8cd52dfb82fdee129e
parent294cf83fda51e9fc2f0c9dca79dda88110637e88 (diff)
downloadlwn-9f0a848f82684f8f1417b3770a679a4441486bf7.tar.gz
lwn-9f0a848f82684f8f1417b3770a679a4441486bf7.zip
module: Fix locking in symbol_put_addr()
commit 275d7d44d802ef271a42dc87ac091a495ba72fc5 upstream. Poma (on the way to another bug) reported an assertion triggering: [<ffffffff81150529>] module_assert_mutex_or_preempt+0x49/0x90 [<ffffffff81150822>] __module_address+0x32/0x150 [<ffffffff81150956>] __module_text_address+0x16/0x70 [<ffffffff81150f19>] symbol_put_addr+0x29/0x40 [<ffffffffa04b77ad>] dvb_frontend_detach+0x7d/0x90 [dvb_core] Laura Abbott <labbott@redhat.com> produced a patch which lead us to inspect symbol_put_addr(). This function has a comment claiming it doesn't need to disable preemption around the module lookup because it holds a reference to the module it wants to find, which therefore cannot go away. This is wrong (and a false optimization too, preempt_disable() is really rather cheap, and I doubt any of this is on uber critical paths, otherwise it would've retained a pointer to the actual module anyway and avoided the second lookup). While its true that the module cannot go away while we hold a reference on it, the data structure we do the lookup in very much _CAN_ change while we do the lookup. Therefore fix the comment and add the required preempt_disable(). Reported-by: poma <pomidorabelisima@gmail.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> Fixes: a6e6abd575fc ("module: remove module_text_address()") Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--kernel/module.c8
1 files changed, 6 insertions, 2 deletions
diff --git a/kernel/module.c b/kernel/module.c
index 1d679a6c942f..49f17c27bf11 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -915,11 +915,15 @@ void symbol_put_addr(void *addr)
if (core_kernel_text(a))
return;
- /* module_text_address is safe here: we're supposed to have reference
- * to module from symbol_get, so it can't go away. */
+ /*
+ * Even though we hold a reference on the module; we still need to
+ * disable preemption in order to safely traverse the data structure.
+ */
+ preempt_disable();
modaddr = __module_text_address(a);
BUG_ON(!modaddr);
module_put(modaddr);
+ preempt_enable();
}
EXPORT_SYMBOL_GPL(symbol_put_addr);