linux-kernel - [PATCH 3.16.y-ckt 66/94] module: Fix locking in symbol_put

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Message-Id: <1447409428-12178-67-git-send-email-luis.henriques@canonical.com>
Date:	Fri, 13 Nov 2015 10:10:00 +0000
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 3.16.y-ckt 66/94] module: Fix locking in symbol_put_addr()

3.16.7-ckt20 -stable 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: Luis Henriques <luis.henriques@...onical.com>
---
 kernel/module.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index fd876c5fcb13..ed4d3b7ec58a 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);
 
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