linux-kernel - Re: [PATCH -next 1/2] blk-iocost: fix divide by 0 error in calc

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Date:   Fri, 25 Nov 2022 10:52:12 +0100
From:   Andreas Herrmann <aherrmann@...e.de>
To:     Li Nan <linan122@...wei.com>
Cc:     tj@...nel.org, josef@...icpanda.com, axboe@...nel.dk,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yukuai3@...wei.com,
        yi.zhang@...wei.com
Subject: Re: [PATCH -next 1/2] blk-iocost: fix divide by 0 error in
 calc_lcoefs()

On Thu, Nov 24, 2022 at 10:06:34PM +0800, Li Nan wrote:
> echo max of u64 to cost.model can cause divide by 0 error.
> 
>   # echo 8:0 rbps=18446744073709551615 > /sys/fs/cgroup/io.cost.model
> 
>   divide error: 0000 [#1] PREEMPT SMP
>   RIP: 0010:calc_lcoefs+0x4c/0xc0
>   Call Trace:
>    <TASK>
>    ioc_refresh_params+0x2b3/0x4f0
>    ioc_cost_model_write+0x3cb/0x4c0
>    ? _copy_from_iter+0x6d/0x6c0
>    ? kernfs_fop_write_iter+0xfc/0x270
>    cgroup_file_write+0xa0/0x200
>    kernfs_fop_write_iter+0x17d/0x270
>    vfs_write+0x414/0x620
>    ksys_write+0x73/0x160
>    __x64_sys_write+0x1e/0x30
>    do_syscall_64+0x35/0x80
>    entry_SYSCALL_64_after_hwframe+0x63/0xcd
> 
> calc_lcoefs() uses the input value of cost.model in DIV_ROUND_UP_ULL,
> overflow would happen if bps plus IOC_PAGE_SIZE is greater than
> ULLONG_MAX, it can cause divide by 0 error.I_LCOEF_MAX is introduced to
> prevent it.
> 
> Signed-off-by: Li Nan <linan122@...wei.com>
> ---
>  block/blk-iocost.c | 5 +++++
>  1 file changed, 5 insertions(+)

Looks good.
Reviewed-by: Andreas Herrmann <aherrmann@...e.de>

> diff --git a/block/blk-iocost.c b/block/blk-iocost.c
> index f01359906c83..a38a5324bf10 100644
> --- a/block/blk-iocost.c
> +++ b/block/blk-iocost.c
> @@ -306,6 +306,9 @@ enum {
>  	IOC_PAGE_SIZE		= 1 << IOC_PAGE_SHIFT,
>  	IOC_SECT_TO_PAGE_SHIFT	= IOC_PAGE_SHIFT - SECTOR_SHIFT,
>  
> +	/* avoid overflow */
> +	I_LCOEF_MAX		= ULLONG_MAX - IOC_PAGE_SIZE,
> +
>  	/* if apart further than 16M, consider randio for linear model */
>  	LCOEF_RANDIO_PAGES	= 4096,
>  };
> @@ -3406,6 +3409,8 @@ static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
>  			goto einval;
>  		if (match_u64(&args[0], &v))
>  			goto einval;
> +		if (v > I_LCOEF_MAX)
> +			goto einval;
>  		u[tok] = v;
>  		user = true;
>  	}
> -- 
> 2.31.1
> 

-- 
Regards,
Andreas

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