linux-kernel - [PATCH] time: create __getnstimeofday for WARNless calls

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Date:	Mon, 19 Nov 2012 10:26:16 -0800
From:	Kees Cook <keescook@...omium.org>
To:	linux-kernel@...r.kernel.org
Cc:	Anton Vorontsov <cbouatmailru@...il.com>,
	Colin Cross <ccross@...roid.com>,
	Kees Cook <keescook@...omium.org>,
	Tony Luck <tony.luck@...el.com>,
	John Stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Anton Vorontsov <anton.vorontsov@...aro.org>
Subject: [PATCH] time: create __getnstimeofday for WARNless calls

The pstore RAM backend can get called during resume, and must be defensive
against a suspended time source. Expose getnstimeofday logic that returns
an error instead of a WARN. This can be detected and the timestamp can
be zeroed out.

Reported-by: Doug Anderson <dianders@...omium.org>
Cc: John Stultz <johnstul@...ibm.com>
Cc: Anton Vorontsov <anton.vorontsov@...aro.org>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 fs/pstore/ram.c           |   10 +++++++---
 include/linux/time.h      |    1 +
 kernel/time/timekeeping.c |   29 ++++++++++++++++++++++++-----
 3 files changed, 32 insertions(+), 8 deletions(-)

diff --git a/fs/pstore/ram.c b/fs/pstore/ram.c
index 1a4f6da..dacfe78 100644
--- a/fs/pstore/ram.c
+++ b/fs/pstore/ram.c
@@ -168,12 +168,16 @@ static ssize_t ramoops_pstore_read(u64 *id, enum pstore_type_id *type,
 static size_t ramoops_write_kmsg_hdr(struct persistent_ram_zone *prz)
 {
 	char *hdr;
-	struct timeval timestamp;
+	struct timespec timestamp;
 	size_t len;
 
-	do_gettimeofday(&timestamp);
+	/* Report zeroed timestamp if called before timekeeping has resumed. */
+	if (__getnstimeofday(&timestamp)) {
+		timestamp.tv_sec = 0;
+		timestamp.tv_nsec = 0;
+	}
 	hdr = kasprintf(GFP_ATOMIC, RAMOOPS_KERNMSG_HDR "%lu.%lu\n",
-		(long)timestamp.tv_sec, (long)timestamp.tv_usec);
+		(long)timestamp.tv_sec, (long)(timestamp.tv_nsec / 1000));
 	WARN_ON_ONCE(!hdr);
 	len = hdr ? strlen(hdr) : 0;
 	persistent_ram_write(prz, hdr, len);
diff --git a/include/linux/time.h b/include/linux/time.h
index 4d358e9..0015aea 100644
--- a/include/linux/time.h
+++ b/include/linux/time.h
@@ -158,6 +158,7 @@ extern int do_setitimer(int which, struct itimerval *value,
 			struct itimerval *ovalue);
 extern unsigned int alarm_setitimer(unsigned int seconds);
 extern int do_getitimer(int which, struct itimerval *value);
+extern int __getnstimeofday(struct timespec *tv);
 extern void getnstimeofday(struct timespec *tv);
 extern void getrawmonotonic(struct timespec *ts);
 extern void getnstime_raw_and_real(struct timespec *ts_raw,
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index e424970..84820dc 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -220,19 +220,18 @@ static void timekeeping_forward_now(struct timekeeper *tk)
 }
 
 /**
- * getnstimeofday - Returns the time of day in a timespec
+ * __getnstimeofday - Returns the time of day in a timespec.
  * @ts:		pointer to the timespec to be set
  *
- * Returns the time of day in a timespec.
+ * Updates the time of day in the timespec.
+ * Returns 0 on success, or -ve when suspended (timespec will be undefined).
  */
-void getnstimeofday(struct timespec *ts)
+int __getnstimeofday(struct timespec *ts)
 {
 	struct timekeeper *tk = &timekeeper;
 	unsigned long seq;
 	s64 nsecs = 0;
 
-	WARN_ON(timekeeping_suspended);
-
 	do {
 		seq = read_seqbegin(&tk->lock);
 
@@ -243,6 +242,26 @@ void getnstimeofday(struct timespec *ts)
 
 	ts->tv_nsec = 0;
 	timespec_add_ns(ts, nsecs);
+
+	/*
+	 * Do not bail out early, in case there were callers still using
+	 * the value, even in the face of the WARN_ON.
+	 */
+	if (unlikely(timekeeping_suspended))
+		return -EAGAIN;
+	return 0;
+}
+EXPORT_SYMBOL(__getnstimeofday);
+
+/**
+ * getnstimeofday - Returns the time of day in a timespec.
+ * @ts:		pointer to the timespec to be set
+ *
+ * Returns the time of day in a timespec (WARN if suspended).
+ */
+void getnstimeofday(struct timespec *ts)
+{
+	WARN_ON(__getnstimeofday(ts));
 }
 EXPORT_SYMBOL(getnstimeofday);
 
-- 
1.7.9.5


-- 
Kees Cook
Chrome OS Security
--
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