[PATCH v2] ntp: safeguard against time_constant overflow case

[Justin Stitt]

Using syzkaller with the recently reintroduced signed integer overflow
sanitizer produces this UBSAN report:

UBSAN: signed-integer-overflow in ../kernel/time/ntp.c:738:18
9223372036854775806 + 4 cannot be represented in type 'long'
Call Trace:
 <TASK>
 dump_stack_lvl+0x93/0xd0
 handle_overflow+0x171/0x1b0
 __do_adjtimex+0x1236/0x1440
 do_adjtimex+0x2be/0x740
 __x64_sys_clock_adjtime+0x154/0x1d0

Rework the logic surrounding time_constant and how it is incremented so
that it doesn't wrap-around.

Link: https://github.com/llvm/llvm-project/pull/82432 [1]
Closes: https://github.com/KSPP/linux/issues/352
Cc: linux-hardening@xxxxxxxxxxxxxxx
Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
---
Changes in v2:
- Adjust commit log (thanks Thomas)
- massively simplify bounds checking for time_constant
- Link to v1: https://lore.kernel.org/r/20240506-b4-sio-ntp-c-v1-1-a01281aa01ba@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, &(0x7f0000000280)={0xff, 0x0, 0x0, 0x0, 0x0, 0x0, 0x7ffffffffffffffe})

.. 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/ntp.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index 406dccb79c2b..d64f69e14938 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -733,8 +733,7 @@ static inline void process_adjtimex_modes(const struct __kernel_timex *txc,
 		time_esterror = txc->esterror;
 
 	if (txc->modes & ADJ_TIMECONST) {
-		time_constant = txc->constant;
-		if (!(time_status & STA_NANO))
+		if (!(time_status & STA_NANO) && time_constant < MAXTC)
 			time_constant += 4;
 		time_constant = min(time_constant, (long)MAXTC);
 		time_constant = max(time_constant, 0l);

---
base-commit: 0106679839f7c69632b3b9833c3268c316c0a9fc
change-id: 20240506-b4-sio-ntp-c-c227b02c65a3

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