Commit 3895ff2d authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ben Hutchings

module: Fix locking in symbol_put_addr()

commit 275d7d44 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: default avatarpoma <pomidorabelisima@gmail.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarRusty Russell <rusty@rustcorp.com.au>
Fixes: a6e6abd5 ("module: remove module_text_address()")
Signed-off-by: default avatarBen Hutchings <ben@decadent.org.uk>
parent 005f90fa
...@@ -893,11 +893,15 @@ void symbol_put_addr(void *addr) ...@@ -893,11 +893,15 @@ void symbol_put_addr(void *addr)
if (core_kernel_text(a)) if (core_kernel_text(a))
return; 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); modaddr = __module_text_address(a);
BUG_ON(!modaddr); BUG_ON(!modaddr);
module_put(modaddr); module_put(modaddr);
preempt_enable();
} }
EXPORT_SYMBOL_GPL(symbol_put_addr); EXPORT_SYMBOL_GPL(symbol_put_addr);
......
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