[S] Change in coreboot[main]: libpayload/libc/time: Fix possible overflow in multiplication

[Yidi Lin (Code Review)]

Yidi Lin has uploaded this change for review. ( https://review.coreboot.org/c/coreboot/+/78888?usp=email )

Change subject: libpayload/libc/time: Fix possible overflow in multiplication
......................................................................

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 1000000. To prevent this, both 1000000 and
hz can be reduced by dividing them by their GCD.

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

Change-Id: Ia55532490651fcf47128b83a8554751f050bcc89
Signed-off-by: Yidi Lin 
---
M payloads/libpayload/libc/time.c
1 file changed, 12 insertions(+), 1 deletion(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/88/78888/1

diff --git a/payloads/libpayload/libc/time.c b/payloads/libpayload/libc/time.c
index 6780008..b6be116 100644
--- a/payloads/libpayload/libc/time.c
+++ b/payloads/libpayload/libc/time.c
@@ -33,6 +33,7 @@

#define __STDC_FORMAT_MACROS

+#include <commonlib/bsd/gcd.h>
 #include <libpayload-config.h>
 #include <libpayload.h>
 #if CONFIG(LP_ARCH_X86) && CONFIG(LP_NVRAM)
@@ -171,6 +172,8 @@
 u64 timer_us(u64 base)
 {
    static u64 hz;
+	static u32 mult;
+	u32 div;

// Only check timer_hz once. Assume it doesn't change.
    if (hz == 0) {
@@ -180,7 +183,15 @@
    		       "must be at least 1MHz.\n", hz);
    		halt();
    	}
+		mult = 1000000;
+		/*
+		 * Bits[63:32] of CNTFRQ_EL0 are reserved. So it is safe to cast
+		 * hz to uint32_t.
+		 */
+		div = gcd32(hz, mult);
+		hz /= div;
+		mult /= div;
    }

-	return (1000000 * timer_raw_value()) / hz - base;
+	return (mult * timer_raw_value()) / hz - base;
 }