[RFC PATCH] timekeeping: Avoid undefined behaviour in 'ktime_get_with_offset()'

[Xiongfeng Wang]

When I ran Syzkaller testsuite, I got the following call trace.
================================================================================
UBSAN: Undefined behaviour in kernel/time/timekeeping.c:801:8
signed integer overflow:
500152103386 + 9223372036854775807 cannot be represented in type 'long long int'
CPU: 6 PID: 13904 Comm: syz-executor.0 Not tainted 4.19.25 #5
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0xca/0x13e lib/dump_stack.c:113
 ubsan_epilogue+0xe/0x81 lib/ubsan.c:159
 handle_overflow+0x193/0x1e2 lib/ubsan.c:190
 ktime_get_with_offset+0x26a/0x2d0 kernel/time/timekeeping.c:801
 common_hrtimer_arm+0x14d/0x220 kernel/time/posix-timers.c:817
 common_timer_set+0x337/0x530 kernel/time/posix-timers.c:863
 do_timer_settime+0x198/0x290 kernel/time/posix-timers.c:892
 __do_sys_timer_settime kernel/time/posix-timers.c:918 [inline]
 __se_sys_timer_settime kernel/time/posix-timers.c:904 [inline]
 __x64_sys_timer_settime+0x18d/0x260 kernel/time/posix-timers.c:904
 do_syscall_64+0xc8/0x580 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x462eb9
Code: f7 d8 64 89 02 b8 ff ff ff ff c3 66 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 bc ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f7968072c58 EFLAGS: 00000246 ORIG_RAX: 00000000000000df
RAX: ffffffffffffffda RBX: 000000000073bf00 RCX: 0000000000462eb9
RDX: 00000000200000c0 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000004 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f79680736bc
R13: 00000000004c54cc R14: 0000000000704278 R15: 00000000ffffffff
================================================================================

It it because global variable 'offsets' is set with a very large but still
valid value. It overflows when we add 'tk->tkr_mono.base' with 'offsets'.

Because 'ktime_get_with_offset()' is a frequently used function, it may
effect the performance if we use 'ktime_add_safe()' to avoid this
undefined behaviour, so we use 'ktime_add_unsafe()' instead.

Signed-off-by: Xiongfeng Wang <wangxiongfeng2@xxxxxxxxx>
---
 kernel/time/timekeeping.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index ac5dbf2..f9c39a6 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -792,7 +792,7 @@ ktime_t ktime_get_with_offset(enum tk_offsets offs)
 
 	do {
 		seq = read_seqcount_begin(&tk_core.seq);
-		base = ktime_add(tk->tkr_mono.base, *offset);
+		base = ktime_add_unsafe(tk->tkr_mono.base, *offset);
 		nsecs = timekeeping_get_ns(&tk->tkr_mono);
 
 	} while (read_seqcount_retry(&tk_core.seq, seq));
-- 
1.7.12.4