[PATCH 3.12 19/72] module: Fix locking in symbol_put_addr()

From: Jiri Slaby
Date: Mon Nov 23 2015 - 08:26:26 EST

From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

3.12-stable review patch. If anyone has any objections, please let me know.


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@xxxxxxxxxx> 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@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Fixes: a6e6abd575fc ("module: remove module_text_address()")
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
kernel/module.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index a97785308f25..3e3f90d82ecc 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -950,11 +950,15 @@ void symbol_put_addr(void *addr)
if (core_kernel_text(a))

- /* 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);
+ preempt_enable();


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