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Mon, 03 Aug 2026 02:14:48 -0700 (PDT) From: Sureshkumar S To: Marc Kleine-Budde , Vincent Mailhol Cc: Oliver Hartkopp , linux-can@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Sureshkumar S Subject: [PATCH net 1/2] can: bittiming: fix divide-by-zero in can_calc_bittiming() Date: Mon, 3 Aug 2026 09:14:25 +0000 Message-ID: <20260803091426.29050-2-ssureshmsd7@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260803091426.29050-1-ssureshmsd7@gmail.com> References: <20260803091426.29050-1-ssureshmsd7@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" can_calc_bittiming() scans the possible time segment combinations and computes the prescaler for each of them as: brp =3D priv->clock.freq / (tsegall * bt->bitrate) bt->bitrate is supplied by userspace via IFLA_CAN_BITTIMING and tsegall * bt->bitrate is a 32 bit multiplication, so the product wraps to zero as soon as bt->bitrate carries enough factors of two for the tsegall values walked by the loop. The division then faults: Oops: divide error: 0000 [#1] SMP KASAN NOPTI RIP: 0010:can_calc_bittiming+0x32e/0xcc0 Call Trace: can_changelink+0x8ba/0x2060 __rtnl_newlink+0x1013/0x18a0 rtnl_newlink+0x6b/0xa0 rtnetlink_rcv_msg+0x6f9/0xb70 netlink_rcv_skb+0x11f/0x350 netlink_unicast+0x5f5/0x860 netlink_sendmsg+0x70a/0xba0 This does not require an absurd bitrate. With the segment limits of a typical controller tsegall reaches 256, so a bitrate of 16777216 is already enough to wrap the product, and that value is below the 20 Mbit/s CAN XL data bitrate ceiling. Any CAN driver providing a bittiming_const is affected; reproduced on dummy_can. Triggering it needs CAP_NET_ADMIN in the netns owning the device. priv->bitrate_max cannot guard against this: it is populated from the optional "max-bitrate" device tree property, so it is zero for most drivers, and can_changelink() only consults it after can_get_bittiming() has already returned. Compute the product with mul_u32_u32(), as can_fixup_bittiming() already does for bt->brp * NSEC_PER_SEC, and divide with div64_u64(). div_u64() cannot be used here because its divisor is a u32, which would truncate the product back to the faulting value. Fixes: 39549eef3587 ("can: CAN Network device driver and Netlink interface") Signed-off-by: Sureshkumar S --- drivers/net/can/dev/calc_bittiming.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/drivers/net/can/dev/calc_bittiming.c b/drivers/net/can/dev/cal= c_bittiming.c index 42498e9d3f38..4809f5e0c96e 100644 --- a/drivers/net/can/dev/calc_bittiming.c +++ b/drivers/net/can/dev/calc_bittiming.c @@ -119,8 +119,12 @@ int can_calc_bittiming(const struct net_device *dev, s= truct can_bittiming *bt, tseg >=3D (btc->tseg1_min + btc->tseg2_min) * 2; tseg--) { tsegall =3D CAN_SYNC_SEG + tseg / 2; =20 - /* Compute all possible tseg choices (tseg=3Dtseg1+tseg2) */ - brp =3D priv->clock.freq / (tsegall * bt->bitrate) + tseg % 2; + /* Compute all possible tseg choices (tseg=3Dtseg1+tseg2). + * A 32 bit tsegall * bt->bitrate can wrap to zero for large + * userspace bitrates, so compute the product in 64 bit. + */ + brp =3D div64_u64(priv->clock.freq, + mul_u32_u32(tsegall, bt->bitrate)) + tseg % 2; 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Mon, 03 Aug 2026 02:14:59 -0700 (PDT) From: Sureshkumar S To: Marc Kleine-Budde , Vincent Mailhol Cc: Oliver Hartkopp , linux-can@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Sureshkumar S Subject: [PATCH net 2/2] can: bittiming: fix bitrate error calculation on unsigned operands Date: Mon, 3 Aug 2026 09:14:26 +0000 Message-ID: <20260803091426.29050-3-ssureshmsd7@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260803091426.29050-1-ssureshmsd7@gmail.com> References: <20260803091426.29050-1-ssureshmsd7@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" can_calc_bittiming() rates each candidate against the requested bitrate with: bitrate_error =3D abs(bt->bitrate - bitrate); Both operands are unsigned int, so the subtraction wraps instead of becoming negative, and abs() resolves an unsigned int argument to its int branch. A wrapped difference is therefore reinterpreted as a small positive value instead of the large error it actually represents. A requested bitrate far above anything the controller can reach then passes the CAN_CALC_MAX_ERROR gate. On a dummy_can device with a 160 MHz clock, requesting 4294967294 bps reports an error of 0.01%, configures 415584 bps and returns success to userspace, where -EINVAL is expected. Use abs_diff(), which subtracts the smaller operand from the larger one and keeps the whole comparison unsigned. Fixes: 7da29f97d6c8 ("can: dev: can-calc-bit-timing(): better sample point = calculation") Signed-off-by: Sureshkumar S --- drivers/net/can/dev/calc_bittiming.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/drivers/net/can/dev/calc_bittiming.c b/drivers/net/can/dev/cal= c_bittiming.c index 4809f5e0c96e..2789b99ab6a8 100644 --- a/drivers/net/can/dev/calc_bittiming.c +++ b/drivers/net/can/dev/calc_bittiming.c @@ -132,7 +132,7 @@ int can_calc_bittiming(const struct net_device *dev, st= ruct can_bittiming *bt, continue; =20 bitrate =3D priv->clock.freq / (brp * tsegall); - bitrate_error =3D abs(bt->bitrate - bitrate); + bitrate_error =3D abs_diff(bt->bitrate, bitrate); =20 /* tseg brp biterror */ if (bitrate_error > best_bitrate_error) --=20 2.43.0