libpayload/libc/time: Fix possible overflow in multiplication

The value from raw_read_cntfrq_el0() could be large enough to cause
overflow when multiplied by USECS_PER_SEC. To prevent this, both
USECS_PER_SEC and hz can be reduced by dividing them by their GCD.

This patch also modifies the return type of `timer_hz()` from
`uint64_t` to `uint32_t`, assuming that in practice the timestamp
counter should never be that fast.

BUG=b:307790895
TEST=boot to kernel and check the timestamps from `cbmem`

Change-Id: Ia55532490651fcf47128b83a8554751f050bcc89
Signed-off-by: Yidi Lin <yidilin@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/78888
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/libpayload/drivers/timer/arm64_arch_timer.c

@@ -32,7 +32,7 @@
 #include <arch/lib_helpers.h>
 #include <libpayload.h>
 
-uint64_t timer_hz(void)
+uint32_t timer_hz(void)
 {
 	return raw_read_cntfrq_el0();
 }

payloads/libpayload/drivers/timer/generic.c

@@ -33,7 +33,7 @@
 #include <assert.h>
 #include <libpayload.h>
 
-uint64_t timer_hz(void)
+uint32_t timer_hz(void)
 {
 	/* libc/time.c currently requires all timers to be at least 1MHz. */
 	assert(CONFIG_LP_TIMER_GENERIC_HZ >= 1000000);

payloads/libpayload/drivers/timer/rdtsc.c

@@ -33,10 +33,12 @@
 
 #include <libpayload.h>
 #include <arch/rdtsc.h>
+#include <assert.h>
 
-uint64_t timer_hz(void)
+uint32_t timer_hz(void)
 {
-	return (uint64_t)lib_sysinfo.cpu_khz * 1000;
+	assert(UINT32_MAX / 1000 >= lib_sysinfo.cpu_khz);
+	return lib_sysinfo.cpu_khz * 1000;
 }
 
 uint64_t timer_raw_value(void)

payloads/libpayload/include/libpayload.h

@@ -519,7 +519,7 @@ void lib_sysinfo_get_memranges(struct memrange **ranges,
 
 /* Timer functions. */
 /* Defined by each architecture. */
-uint64_t timer_hz(void);
+uint32_t timer_hz(void);
 uint64_t timer_raw_value(void);
 uint64_t timer_us(uint64_t base);
 /* Generic. */

payloads/libpayload/libc/time.c

@@ -38,6 +38,7 @@
 #if CONFIG(LP_ARCH_X86) && CONFIG(LP_NVRAM)
 #include <arch/rdtsc.h>
 #endif
+#include <commonlib/bsd/gcd.h>
 #include <inttypes.h>
 
 extern u32 cpu_khz;
@@ -170,17 +171,21 @@ void arch_ndelay(uint64_t ns)
 
 u64 timer_us(u64 base)
 {
-	static u64 hz;
+	static u32 hz, mult = USECS_PER_SEC;
+	u32 div;
 
 	// Only check timer_hz once. Assume it doesn't change.
 	if (hz == 0) {
 		hz = timer_hz();
-		if (hz < 1000000) {
-			printf("Timer frequency %" PRIu64 " is too low, "
+		if (hz < mult) {
+			printf("Timer frequency %" PRIu32 " is too low, "
 			       "must be at least 1MHz.\n", hz);
 			halt();
 		}
+		div = gcd32(hz, mult);
+		hz /= div;
+		mult /= div;
 	}
 
-	return (1000000 * timer_raw_value()) / hz - base;
+	return (mult * timer_raw_value()) / hz - base;
 }