[PATCH v4] timekeeping: Use READ_ONCE/WRITE_ONCE for xtime_sec to prevent tearing

[Dennis Moshegov <dennis@xzync.uk>]

The timekeeper update path uses a bulk memcpy() to synchronize the timekeeper structure, which is not guaranteed to be atomic. This allows for torn reads in ktime_get_real_seconds() on 64-bit systems, where the sequence counter protection is bypassed for performance.

To prevent reading a torn 64-bit xtime_sec value, enforce atomic-like access by using WRITE_ONCE() for the critical field before the bulk memcpy() in timekeeping_update_from_shadow(). Correspondingly, use READ_ONCE() in ktime_get_real_seconds() to ensure a fresh, consistent load from memory.

Reported-by: syzbot+72789cd1697965e714ca@syzkaller.appspotmail.com

Closes: https://syzkaller.appspot.com/bug?extid=72789cd1697965e714ca
Signed-off-by: Dennis Moshegov <dennis@xzync.uk>
---
 kernel/time/timekeeping.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index b1b5ec43c0f2..bb1c020c0e8b 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -832,6 +832,7 @@ static void timekeeping_update_from_shadow(struct tk_data *tkd, unsigned int act
 	 * the cacheline optimized data layout of the timekeeper and requires
 	 * another indirection.
 	 */
+	WRITE_ONCE(tkd->timekeeper.xtime_sec, tk->xtime_sec);
 	memcpy(&tkd->timekeeper, tk, sizeof(*tk));
 	write_seqcount_end(&tkd->seq);
 }
@@ -1159,11 +1160,11 @@ time64_t ktime_get_real_seconds(void)
 	unsigned int seq;
 
 	if (IS_ENABLED(CONFIG_64BIT))
-		return tk->xtime_sec;
+		return READ_ONCE(tk->xtime_sec);
 
 	do {
 		seq = read_seqcount_begin(&tk_core.seq);
-		seconds = tk->xtime_sec;
+		seconds = READ_ONCE(tk->xtime_sec);
 
 	} while (read_seqcount_retry(&tk_core.seq, seq));
 
@@ -1185,7 +1186,7 @@ noinstr time64_t __ktime_get_real_seconds(void)
 {
 	struct timekeeper *tk = &tk_core.timekeeper;
 
-	return tk->xtime_sec;
+	return READ_ONCE(tk->xtime_sec);
 }
 
 static inline u64 tk_clock_read_snapshot(const struct tk_read_base *tkr,
-- 
2.53.0.windows.1