[PATCH v3] posix-cpu-timers: Avoid undefined behaviour in timespec64_to_ns()

[Xiongfeng Wang]

When I ran Syzkaller testsuite, I got the following call trace.
================================================================================
UBSAN: Undefined behaviour in ./include/linux/time64.h:120:27
signed integer overflow:
8243129037239968815 * 1000000000 cannot be represented in type 'long long int'
CPU: 5 PID: 28854 Comm: syz-executor.1 Not tainted 4.19.24 #4
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.0-0-g63451fca13-prebuilt.qemu-project.org 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
 timespec64_to_ns include/linux/time64.h:120 [inline]
 posix_cpu_timer_set+0x95a/0xb70 kernel/time/posix-cpu-timers.c:687
 do_timer_settime+0x198/0x2a0 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:00007f14e4127c58 EFLAGS: 00000246 ORIG_RAX: 00000000000000df
RAX: ffffffffffffffda RBX: 000000000073bfa0 RCX: 0000000000462eb9
RDX: 0000000020000080 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000004 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f14e41286bc
R13: 00000000004c54cc R14: 0000000000704278 R15: 00000000ffffffff
================================================================================

It is because 'it_interval.tv_sec' is larger than 'KTIME_SEC_MAX' and
'it_interval.tv_sec * NSEC_PER_SEC' overflows in 'timespec64_to_ns()'.

This patch checks whether 'it_interval.tv_sec' is larger than
'KTIME_SEC_MAX' and saturate if that is the case.

Signed-off-by: Xiongfeng Wang <wangxiongfeng2@xxxxxxxxxx>

---
v3:
  - add limit check and saturate
v2:
  - use 'timespec64_valid_restrict()' to check the input and return
    -EINVAL if check fails.
---
 kernel/time/posix-timers.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 0e84bb7..4b57566 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -856,6 +856,10 @@ static int do_timer_settime(timer_t timer_id, int flags,
 	if (!timespec64_valid(&new_spec64->it_interval) ||
 	    !timespec64_valid(&new_spec64->it_value))
 		return -EINVAL;
+	if (new_spec64->it_interval.tv_sec > KTIME_SEC_MAX)
+		new_spec64->it_interval.tv_sec = KTIME_SEC_MAX;
+	if (new_spec64->it_value.tv_sec > KTIME_SEC_MAX)
+		new_spec64->it_value.tv_sec = KTIME_SEC_MAX;
 
 	if (old_spec64)
 		memset(old_spec64, 0, sizeof(*old_spec64));
-- 
1.7.12.4