Commit 275d7d44 authored Aug 20, 2015 by Peter Zijlstra Committed by Rusty Russell Aug 24, 2015

module: Fix locking in symbol_put_addr()



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: a6e6abd5 ("module: remove module_text_address()")
Cc: stable@kernel.org

parent 1b647a16

kernel/module.c

+6 −2

Original line number	Diff line number	Diff line
		@@ -1063,11 +1063,15 @@ 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);

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