linux-kernel - [PATCH] ntp: safeguard against time

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Date: Mon, 06 May 2024 22:01:49 +0000
From: Justin Stitt <justinstitt@...gle.com>
To: John Stultz <jstultz@...gle.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Stephen Boyd <sboyd@...nel.org>, Nathan Chancellor <nathan@...nel.org>, Bill Wendling <morbo@...gle.com>
Cc: linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, 
	Justin Stitt <justinstitt@...gle.com>
Subject: [PATCH] ntp: safeguard against time_constant overflow case

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

[   46.809326] ------------[ cut here ]------------
[   46.812882] UBSAN: signed-integer-overflow in ../kernel/time/ntp.c:738:18
[   46.817676] 9223372036854775806 + 4 cannot be represented in type 'long'
[   46.822346] CPU: 1 PID: 685 Comm: syz-executor.0 Not tainted 6.8.0-rc2-00036-g679ee73ec453 #2
[   46.828270] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[   46.834836] Call Trace:
[   46.836625]  <TASK>
[   46.838147]  dump_stack_lvl+0x93/0xd0
[   46.840771]  handle_overflow+0x171/0x1b0
[   46.843516]  __do_adjtimex+0x1236/0x1440
[   46.846275]  do_adjtimex+0x2be/0x740
[   46.848864]  __x64_sys_clock_adjtime+0x154/0x1d0
[   46.852164]  do_syscall_64+0xd7/0x1b0
[   46.854783]  ? arch_exit_to_user_mode_prepare+0x11/0x60
[   46.858426]  entry_SYSCALL_64_after_hwframe+0x6f/0x77
[   46.861914] RIP: 0033:0x7fde90aaf539
[   46.864500] Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 f1 14 00 00 90 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 8
[   46.877151] RSP: 002b:00007ffebfe63358 EFLAGS: 00000246 ORIG_RAX: 0000000000000131
[   46.882279] RAX: ffffffffffffffda RBX: 00007fde90be3f80 RCX: 00007fde90aaf539
[   46.887270] RDX: 0000000000000000 RSI: 0000000020000280 RDI: 0000000000000000
[   46.892174] RBP: 00007fde90b0e496 R08: 0000000000000000 R09: 0000000000000000
[   46.897061] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
[   46.902020] R13: 0000000000000095 R14: 00007fde90be3f80 R15: 00007fde90be3f80
[   46.906946]  </TASK>
[   46.908537] ---[ end trace ]---

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 being re-enabled in the
kernel with Commit 557f8c582a9ba8ab ("ubsan: Reintroduce signed overflow
sanitizer").

Nonetheless, let's slightly rework the logic surrounding time_constant
and how it is incremented such that we avoid unintentional wrap-around
(even though it is extremely unlikely to be hit in non-fuzzing scenarios).

[1]: https://github.com/llvm/llvm-project/pull/82432

Signed-off-by: Justin Stitt <justinstitt@...gle.com>
---
 kernel/time/ntp.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index 406dccb79c2b..a9f039601968 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -65,6 +65,9 @@ static s64			time_offset;
 /* pll time constant:							*/
 static long			time_constant = 2;
 
+/* pll time constant increment:						*/
+static long			time_constant_inc = 4;
+
 /* maximum error (usecs):						*/
 static long			time_maxerror = NTP_PHASE_LIMIT;
 
@@ -734,10 +737,10 @@ static inline void process_adjtimex_modes(const struct __kernel_timex *txc,
 
 	if (txc->modes & ADJ_TIMECONST) {
 		time_constant = txc->constant;
-		if (!(time_status & STA_NANO))
-			time_constant += 4;
-		time_constant = min(time_constant, (long)MAXTC);
-		time_constant = max(time_constant, 0l);
+		if (!(time_status & STA_NANO) &&
+		    unlikely(LONG_MAX - time_constant_inc >= time_constant))
+			time_constant += time_constant_inc;
+		time_constant = clamp_t(long, time_constant, 0, MAXTC);
 	}
 
 	if (txc->modes & ADJ_TAI &&

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

Best regards,
--
Justin Stitt <justinstitt@...gle.com>