linux-kernel - [PATCH 17/17] x86/tsc: In read_tsc, use rdtsc_ordered() instead of get

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

Message-Id: <66eff21ac4e8133d05f8e5aeff899510c9be1493.1434152192.git.luto@kernel.org>
Date:	Fri, 12 Jun 2015 16:41:52 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	x86@...nel.org
Cc:	Borislav Petkov <bp@...e.de>,
	Peter Zijlstra <peterz@...radead.org>,
	John Stultz <john.stultz@...aro.org>,
	linux-kernel@...r.kernel.org, Len Brown <lenb@...nel.org>,
	Huang Rui <ray.huang@....com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH 17/17] x86/tsc: In read_tsc, use rdtsc_ordered() instead of get_cycles()

There are two logical changes here.  First, this removes a check for
cpu_has_tsc.  That check is unnecessary, as we don't register the
TSC as a clocksource on systems that have no TSC.  Second, it adds a
barrier, thus preventing observable non-monotonicity.

I suspect that the missing barrier was never a problem in practice
because system calls themselves were heavy enough barriers to
prevent user code from observing time warps due to speculation.
(Without the corresponding barrier in the vDSO, however,
non-monotonicity is easy to detect.)

Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/kernel/tsc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 5eb4d91ce9db..c3b6c04c5bf7 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -961,7 +961,7 @@ static struct clocksource clocksource_tsc;
  */
 static cycle_t read_tsc(struct clocksource *cs)
 {
-	return (cycle_t)get_cycles();
+	return (cycle_t)rdtsc_ordered();
 }
 
 /*
-- 
2.4.2

--
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/