x86/vdso: Make delta calculation overflow safe

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. Select GENERIC_VDSO_OVERFLOW_PROTECT so that
max_cycles is made available in the VDSO data page. Check against
max_cycles, falling back to a slower higher precision calculation. Take
advantage of the opportunity to move masking and negative motion check
into the slow path.

The result is a calculation that has similar performance as before. Newer
machines showed performance benefit, whereas older Skylake-based hardware
such as Intel Kaby Lake was seen <1% worse.
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240325064023.2997-9-adrian.hunter@intel.com

arch/x86/Kconfig

@@ -168,6 +168,7 @@ config X86
 	select GENERIC_TIME_VSYSCALL
 	select GENERIC_GETTIMEOFDAY
 	select GENERIC_VDSO_TIME_NS
+	select GENERIC_VDSO_OVERFLOW_PROTECT
 	select GUP_GET_PXX_LOW_HIGH		if X86_PAE
 	select HARDIRQS_SW_RESEND
 	select HARDLOCKUP_CHECK_TIMESTAMP	if X86_64

arch/x86/include/asm/vdso/gettimeofday.h

@@ -319,18 +319,31 @@ static inline bool arch_vdso_cycles_ok(u64 cycles)
  */
 static __always_inline u64 vdso_calc_ns(const struct vdso_data *vd, u64 cycles, u64 base)
 {
-	/*
-	 * Due to the MSB/Sign-bit being used as invalid marker (see
-	 * arch_vdso_cycles_valid() above), the effective mask is S64_MAX.
-	 */
-	u64 delta = (cycles - vd->cycle_last) & S64_MAX;
+	u64 delta = cycles - vd->cycle_last;
 
 	/*
-	 * Due to the above mentioned TSC wobbles, filter out negative motion.
-	 * Per the above masking, the effective sign bit is now bit 62.
+	 * Negative motion and deltas which can cause multiplication
+	 * overflow require special treatment. This check covers both as
+	 * negative motion is guaranteed to be greater than @vd::max_cycles
+	 * due to unsigned comparison.
+	 *
+	 * Due to the MSB/Sign-bit being used as invalid marker (see
+	 * arch_vdso_cycles_valid() above), the effective mask is S64_MAX,
+	 * but that case is also unlikely and will also take the unlikely path
+	 * here.
 	 */
-	if (unlikely(delta & (1ULL << 62)))
-		return base >> vd->shift;
+	if (unlikely(delta > vd->max_cycles)) {
+		/*
+		 * Due to the above mentioned TSC wobbles, filter out
+		 * negative motion.  Per the above masking, the effective
+		 * sign bit is now bit 62.
+		 */
+		if (delta & (1ULL << 62))
+			return base >> vd->shift;
+
+		/* Handle multiplication overflow gracefully */
+		return mul_u64_u32_add_u64_shr(delta & S64_MAX, vd->mult, base, vd->shift);
+	}
 
 	return ((delta * vd->mult) + base) >> vd->shift;
 }