[PATCH v2] ntp: remove accidental integer wrap-around

[Justin Stitt]

Using syzkaller alongside the newly reintroduced signed integer overflow
sanitizer spits out this report:

UBSAN: signed-integer-overflow in ../kernel/time/ntp.c:461:16
9223372036854775807 + 500 cannot be represented in type 'long'
Call Trace:
 <IRQ>
 dump_stack_lvl+0x93/0xd0
 handle_overflow+0x171/0x1b0
 second_overflow+0x2d6/0x500
 accumulate_nsecs_to_secs+0x60/0x160
 timekeeping_advance+0x1fe/0x890
 update_wall_time+0x10/0x30
..

time_maxerror is unconditionally incremented and the result is checked
against NTP_PHASE_LIMIT, but the increment itself can overflow,
resulting in wrap-around to negative space.

The user can supply some crazy values which is causing the overflow. Add
an extra validation step checking that maxerror is reasonable.

Link: https://github.com/llvm/llvm-project/pull/82432 [1]
Closes: https://github.com/KSPP/linux/issues/354
Cc: linux-hardening@xxxxxxxxxxxxxxx
Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
---
Changes in v2:
- update commit log (thanks Thomas)
- check for sane user input during validation (thanks Thomas)
- Link to v1: https://lore.kernel.org/r/20240507-b4-sio-ntp-usec-v1-1-15003fc9c2b4@xxxxxxxxxx
---

Historically, the signed integer overflow sanitizer did not work in the
kernel due to its interaction with `-fwrapv` but this has since been
changed [1] in the newest version of Clang. It was re-enabled in the
kernel with Commit 557f8c582a9ba8ab ("ubsan: Reintroduce signed overflow
sanitizer").

Here's the syzkaller reproducer:
| #{Threaded:false Repeat:false RepeatTimes:0 Procs:1 Slowdown:1 Sandbox:
| #SandboxArg:0 Leak:false NetInjection:false NetDevices:false
| #NetReset:false Cgroups:false BinfmtMisc:false CloseFDs:false KCSAN:false
| #DevlinkPCI:false NicVF:false USB:false VhciInjection:false Wifi:false
| #IEEE802154:false Sysctl:false Swap:false UseTmpDir:false
| #HandleSegv:false Repro:false Trace:false LegacyOptions:{Collide:false
| #Fault:false FaultCall:0 FaultNth:0}}
| clock_adjtime(0x0, &(0x7f0000000000)={0x5, 0x1, 0x40,
| 0x7fffffffffffffff, 0x8, 0xb2, 0x256, 0x6, 0x5, 0x8001, 0x9, 0x3f, 0x0,
| 0x8000, 0x800, 0x64d, 0x50000, 0x7ff, 0x8000000000000001, 0x1f, 0x3,
| 0xfff, 0x7fffffff, 0x5, 0x100, 0x4})

.. which was used against Kees' tree here (v6.8rc2):
https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=wip/v6.9-rc2/unsigned-overflow-sanitizer

.. with this config:
https://gist.github.com/JustinStitt/824976568b0f228ccbcbe49f3dee9bf4
---
 kernel/time/timekeeping.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index b58dffc58a8f..321f251c02aa 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -2388,6 +2388,11 @@ static int timekeeping_validate_timex(const struct __kernel_timex *txc)
 		}
 	}
 
+	if (txc->modes & ADJ_MAXERROR) {
+		if (txc->maxerror < 0 || txc->maxerror > NTP_PHASE_LIMIT)
+			return -EINVAL;
+	}
+
 	/*
 	 * Check for potential multiplication overflows that can
 	 * only happen on 64-bit systems:

---
base-commit: 0106679839f7c69632b3b9833c3268c316c0a9fc
change-id: 20240507-b4-sio-ntp-usec-1a3ab67bdce1

Best regards,
--
Justin Stitt <justinstitt@xxxxxxxxxx>