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Date:	Sun, 15 Nov 2015 01:45:45 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "poma" <pomidorabelisima@...il.com>,
	"Peter Zijlstra" <peterz@...radead.org>,
	"Rusty Russell" <rusty@...tcorp.com.au>
Subject: [PATCH 3.2 02/60] module: Fix locking in symbol_put_addr()

3.2.73-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Peter Zijlstra <peterz@...radead.org>

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@...hat.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@...il.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
Fixes: a6e6abd575fc ("module: remove module_text_address()")
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 kernel/module.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/kernel/module.c
+++ b/kernel/module.c
@@ -893,11 +893,15 @@ 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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