From nobody Fri Jul 24 21:29:12 2026 Received: from mail-wm1-f49.google.com (mail-wm1-f49.google.com [209.85.128.49]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0E079370AF0 for ; Fri, 24 Jul 2026 15:44:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.128.49 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784907880; cv=none; b=DctOjrXiMXDzC5wlUc2JZb8VEEzMYM348D0F9aVJ6X9rryJXda3VmYDaVgaNhGljyD1PnG0S3xDWl3B0lsqJv2o8LuspLtd1hOh6q1onKmUA7bIK3Ih4NX8V5IQ3BhmpGSq1eTxWc4GeH9FB4fztj2ze4dqFMf0i9AQB8Ormc5w= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784907880; c=relaxed/simple; bh=DH57JbfQdMHDNOM4p6fx+GnMQi+2DajhrRsnUdZnUQY=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=fJ83Y8JLI0IsFcSptmC061mMZq5StE6sZGVswOxF1z5eDlPMVoqpDAdoYPKaSVkIejrAMVr7lu0VoNWLSVGcWnb6a1K7l6jH/Lh6QFf2FjP27OXsRn0O+nYFbs1oWkRLu9IFJICrl5UJyIsZBP5UD/ODjtD937j6fF43NKODcc8= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=fail (p=none dis=none) header.from=xzync.uk; spf=pass smtp.mailfrom=gmail.com; arc=none smtp.client-ip=209.85.128.49 Authentication-Results: smtp.subspace.kernel.org; dmarc=fail (p=none dis=none) header.from=xzync.uk Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Received: by mail-wm1-f49.google.com with SMTP id 5b1f17b1804b1-495757ccbc1so5640135e9.2 for ; Fri, 24 Jul 2026 08:44:38 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1784907877; x=1785512677; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:x-gm-gg:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to:content-type; bh=ZhiIGx2k+EPnKXUbmGoJZp074o95j6s9ns1Hb6L3/A4=; b=p21yc4srPNW3MXN87j5eG6AUbOudtPZnUZKhWOXWQnXpBco/x4EK7u727umXJuca0c OW4c5O9lW2k1vYikDsDZzeVTJgZOcF7SPAuhHZMGxmTvG5Kelg/HBlkzrekZuA+Zm8Ul tgS4iU3cOG6ixtYly8njskts/p8/4LNn6qx5ibK9vRGrPbYfFeyN8LzmkBdhUKyPzAYy 2DZctD3asm/i2SxM8cZy+sxpcYumxO59XTAt6/enEo2to+uUgt7efuIQ9lgjOJyEbEvW WauY9mXnivTD573pbbei5la4QESIi9iUR/tf6QN7NSst8BhomlIc6gMr+BW+Y/TYdsmv 8Nhw== X-Gm-Message-State: AOJu0YxEK2VWxlgJBAmo6ZNMvAGd1Sv+IYY1x1WJj3XM3tPucIbsCHXr BmzHxE6Qsq27cbBXOUvO84uHvp1alARP8ftOh8MPgpuIAbAyIhZtbaHX9h/rQOQxEotUHA== X-Gm-Gg: AR+sD13knTNGuYiGmrgnKEksNVL3cP40MXsn1f/LbaOC2NWVIJtORcSq5f4quXznuSq CYmKfjyRPducjhfyCSErcn9XQChB32wny4XEFKSYC1YEIK8JL8hXzfVa+LQiw3LAETSxcsfQucP N4GNVDu+Veej3+pdq1C+QEBy6ff69pc35FbimIeLkEsWYMZdQmma96FsWksesDuZt4Swhqq1BwI bAttnfcDTdTfabgh3mK3wflA4507GeEjCJDvwWP4wXYHJMxjyl40dMQzzPHmeJwJftC3aIGF0fO Gdg2i0f8j4VyZWBT69RQPgrOWK3pfrWB7kCe7gQAbpGKotKRRbLp+8tnyoYEzG+g83GYJw/hI3f JaFm37WS7W0VosVFVnwp0NCtWhMGURRYxbupUtNADHnmd9wmyyyQfe+bBIfXibHAyN6GFVd/Hfh d2Du7UIH+lDXUF3+QmkIFB9lFImPF6AwiQ5RcWkeT0 X-Received: by 2002:a05:600c:6610:b0:495:6a2a:951f with SMTP id 5b1f17b1804b1-49573cd02fdmr92910445e9.17.1784907877164; Fri, 24 Jul 2026 08:44:37 -0700 (PDT) Received: from DESKTOP-9PKP46A ([2a0a:ef40:e0a:5301:54ee:55e5:c9ee:66ed]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4957af6ad58sm91437985e9.4.2026.07.24.08.44.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 24 Jul 2026 08:44:36 -0700 (PDT) From: Dennis Moshegov To: tglx@linutronix.de Cc: linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, Dennis Moshegov , syzbot+72789cd1697965e714ca@syzkaller.appspotmail.com Subject: [PATCH v4] timekeeping: Use READ_ONCE/WRITE_ONCE for xtime_sec to prevent tearing Date: Fri, 24 Jul 2026 16:43:55 +0100 Message-ID: <20260724154405.70-1-dennis@xzync.uk> X-Mailer: git-send-email 2.53.0.windows.1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" The timekeeper update path uses a bulk memcpy() to synchronize the timekeep= er structure, which is not guaranteed to be atomic. This allows for torn re= ads in ktime_get_real_seconds() on 64-bit systems, where the sequence count= er protection is bypassed for performance. To prevent reading a torn 64-bit xtime_sec value, enforce atomic-like acces= s 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=3D72789cd1697965e714ca Signed-off-by: Dennis Moshegov --- 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_da= ta *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; =20 if (IS_ENABLED(CONFIG_64BIT)) - return tk->xtime_sec; + return READ_ONCE(tk->xtime_sec); =20 do { seq =3D read_seqcount_begin(&tk_core.seq); - seconds =3D tk->xtime_sec; + seconds =3D READ_ONCE(tk->xtime_sec); =20 } while (read_seqcount_retry(&tk_core.seq, seq)); =20 @@ -1185,7 +1186,7 @@ noinstr time64_t __ktime_get_real_seconds(void) { struct timekeeper *tk =3D &tk_core.timekeeper; =20 - return tk->xtime_sec; + return READ_ONCE(tk->xtime_sec); } =20 static inline u64 tk_clock_read_snapshot(const struct tk_read_base *tkr, --=20 2.53.0.windows.1