Received-SPF: pass (google.com: domain of linux-kernel+bounces-116596-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
From: Adrian Hunter <adrian.hunter@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Michael Ellerman <mpe@ellerman.id.au>,
	Nicholas Piggin <npiggin@gmail.com>,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
	"Aneesh Kumar K.V" <aneesh.kumar@kernel.org>,
	"Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
	Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Alexander Gordeev <agordeev@linux.ibm.com>,
	Christian Borntraeger <borntraeger@linux.ibm.com>,
	Sven Schnelle <svens@linux.ibm.com>,
	Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org,
	"H. Peter Anvin" <hpa@zytor.com>,
	Andy Lutomirski <luto@kernel.org>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	John Stultz <jstultz@google.com>,
	Stephen Boyd <sboyd@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Anna-Maria Behnsen <anna-maria@linutronix.de>,
	linuxppc-dev@lists.ozlabs.org,
	linux-kernel@vger.kernel.org,
	linux-s390@vger.kernel.org
Subject: [PATCH V2 07/19] vdso: Make delta calculation overflow safe
Date: Mon, 25 Mar 2024 08:40:11 +0200
Message-Id: <20240325064023.2997-8-adrian.hunter@intel.com>
In-Reply-To: <20240325064023.2997-1-adrian.hunter@intel.com>
References: <20240325064023.2997-1-adrian.hunter@intel.com>
Precedence: bulk
MIME-Version: 1.0
Organization: Intel Finland Oy, Registered Address: PL 281, 00181 Helsinki, Business Identity Code: 0357606 - 4, Domiciled in Helsinki
Content-Transfer-Encoding: 8bit

Kernel timekeeping is designed to keep the change in cycles (since the last
timer interrupt) below max_cycles, which prevents multiplication overflow
when converting cycles to nanoseconds. However, if timer interrupts stop,
the calculation will eventually overflow.

Add protection against that, enabled by config option
CONFIG_GENERIC_VDSO_OVERFLOW_PROTECT. Check against max_cycles, falling
back to a slower higher precision calculation.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
---
 lib/vdso/gettimeofday.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
index 9fa90e0794c9..9c3a8d2440c9 100644
--- a/lib/vdso/gettimeofday.c
+++ b/lib/vdso/gettimeofday.c
@@ -13,6 +13,18 @@
 # define VDSO_DELTA_MASK(vd)	(vd->mask)
 #endif
 
+#ifdef CONFIG_GENERIC_VDSO_OVERFLOW_PROTECT
+static __always_inline bool vdso_delta_ok(const struct vdso_data *vd, u64 delta)
+{
+	return delta < vd->max_cycles;
+}
+#else
+static __always_inline bool vdso_delta_ok(const struct vdso_data *vd, u64 delta)
+{
+	return true;
+}
+#endif
+
 #ifndef vdso_shift_ns
 static __always_inline u64 vdso_shift_ns(u64 ns, u32 shift)
 {
@@ -28,7 +40,10 @@ static __always_inline u64 vdso_calc_ns(const struct vdso_data *vd, u64 cycles,
 {
 	u64 delta = (cycles - vd->cycle_last) & VDSO_DELTA_MASK(vd);
 
-	return vdso_shift_ns((delta * vd->mult) + base, vd->shift);
+	if (likely(vdso_delta_ok(vd, delta)))
+		return vdso_shift_ns((delta * vd->mult) + base, vd->shift);
+
+	return mul_u64_u32_add_u64_shr(delta, vd->mult, base, vd->shift);
 }
 #endif /* vdso_calc_ns */
 
-- 
2.34.1