linux-kernel - [PATCH] Immunize rcu_dereference() against crazy compiler writers

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <20070712010058.GA30869@linux.vnet.ibm.com>
Date:	Wed, 11 Jul 2007 18:00:58 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...e.hu, dipankar@...ibm.com, josht@...ux.vnet.ibm.com,
	akpm@...ux-foundation.org
Subject: [PATCH] Immunize rcu_dereference() against crazy compiler writers

Turns out that compiler writers are a bit more aggressive about optimizing
than one might expect.  This patch prevents a number of such optimizations
from messing up rcu_deference().  This is not merely a theoretical
problem, as evidenced by the rmb() in mce_log().

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---

 rcupdate.h |   14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff -urpNa -X dontdiff linux-2.6.22/include/linux/rcupdate.h linux-2.6.22-volrcud/include/linux/rcupdate.h
--- linux-2.6.22/include/linux/rcupdate.h	2007-07-08 16:32:17.000000000 -0700
+++ linux-2.6.22-volrcud/include/linux/rcupdate.h	2007-07-11 17:21:09.000000000 -0700
@@ -217,6 +217,18 @@ extern int rcu_needs_cpu(int cpu);
 		local_bh_enable(); \
 	} while(0)
 
+/*
+ * Prevent the compiler from merging or refetching accesses.  The compiler
+ * is also forbidden from reordering successive instances of ACCESS_ONCE(),
+ * but only when the compiler is aware of some particular ordering.  One way
+ * to make the compiler aware of ordering is to put the two invocations of 
+ * ACCESS_ONCE() in different C statements.
+ *
+ * This macro does absolutely -nothing- to prevent the CPU from reordering,
+ * merging, or refetching absolutely anything at any time.
+ */
+#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
+
 /**
  * rcu_dereference - fetch an RCU-protected pointer in an
  * RCU read-side critical section.  This pointer may later
@@ -228,7 +240,7 @@ extern int rcu_needs_cpu(int cpu);
  */
 
 #define rcu_dereference(p)     ({ \
-				typeof(p) _________p1 = p; \
+				typeof(p) _________p1 = ACCESS_ONCE(p); \
 				smp_read_barrier_depends(); \
 				(_________p1); \
 				})
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/