RE: [PATCH][next] ixgbe: fix potential u32 overflow on shift

[Keller, Jacob E]

> -----Original Message-----
> From: Colin King [mailto:colin.king@xxxxxxxxxxxxx]
> Sent: Thursday, June 06, 2019 6:11 AM
> To: Keller, Jacob E <jacob.e.keller@xxxxxxxxx>; Kirsher, Jeffrey T
> <jeffrey.t.kirsher@xxxxxxxxx>; David S . Miller <davem@xxxxxxxxxxxxx>; intel-wired-
> lan@xxxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx
> Cc: kernel-janitors@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: [PATCH][next] ixgbe: fix potential u32 overflow on shift
> 
> From: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> 
> The u32 variable rem is being shifted using u32 arithmetic however
> it is being passed to div_u64 that expects the expression to be a u64.
> The 32 bit shift may potentially overflow, so cast rem to a u64 before
> shifting to avoid this.
> 
> Addresses-Coverity: ("Unintentional integer overflow")
> Fixes: cd4583206990 ("ixgbe: implement support for SDP/PPS output on X550
> hardware")
> Fixes: 68d9676fc04e ("ixgbe: fix PTP SDP pin setup on X540 hardware")
> Signed-off-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> ---
>  drivers/net/ethernet/intel/ixgbe/ixgbe_ptp.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_ptp.c
> b/drivers/net/ethernet/intel/ixgbe/ixgbe_ptp.c
> index 2c4d327fcc2e..ff229d0e9146 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_ptp.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_ptp.c
> @@ -209,7 +209,7 @@ static void ixgbe_ptp_setup_sdp_X540(struct ixgbe_adapter
> *adapter)
>  	 * assumes that the cycle counter shift is small enough to avoid
>  	 * overflowing when shifting the remainder.
>  	 */

With this change, the comment above the div_u64 doesn't make much sense. I would also drop the part about the assuming it won't overflow the remainder.

> -	clock_edge += div_u64((rem << cc->shift), cc->mult);
> +	clock_edge += div_u64(((u64)rem << cc->shift), cc->mult);
>  	trgttiml = (u32)clock_edge;
>  	trgttimh = (u32)(clock_edge >> 32);
> 
> @@ -295,7 +295,7 @@ static void ixgbe_ptp_setup_sdp_X550(struct ixgbe_adapter
> *adapter)
>  	 * assumes that the cycle counter shift is small enough to avoid
>  	 * overflowing when shifting the remainder.
>  	 */

Same here.

Thanks,
Jake

> -	clock_edge += div_u64((rem << cc->shift), cc->mult);
> +	clock_edge += div_u64(((u64)rem << cc->shift), cc->mult);
> 
>  	/* X550 hardware stores the time in 32bits of 'billions of cycles' and
>  	 * 32bits of 'cycles'. There's no guarantee that cycles represents
> --
> 2.20.1