linux-kernel - Re: [RESEND PATCH] libfs: fix error cast of negative value in simple_attr

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Message-Id: <20201110111842.1bc76e9def94279d4453ff67@linux-foundation.org>
Date:   Tue, 10 Nov 2020 11:18:42 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Yicong Yang <yangyicong@...ilicon.com>
Cc:     <viro@...iv.linux.org.uk>, <linux-fsdevel@...r.kernel.org>,
        <akinobu.mita@...il.com>, <linux-kernel@...r.kernel.org>,
        <linuxarm@...wei.com>, <prime.zeng@...wei.com>
Subject: Re: [RESEND PATCH] libfs: fix error cast of negative value in
 simple_attr_write()

On Tue, 10 Nov 2020 17:25:24 +0800 Yicong Yang <yangyicong@...ilicon.com> wrote:

> The attr->set() receive a value of u64, but simple_strtoll() is used
> for doing the conversion. It will lead to the error cast if user inputs
> a negative value.
> 
> Use kstrtoull() instead of simple_strtoll() to convert a string got
> from the user to an unsigned value. The former will return '-EINVAL' if
> it gets a negetive value, but the latter can't handle the situation
> correctly.
> 
> ...
>
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -977,7 +977,9 @@ ssize_t simple_attr_write(struct file *file, const char __user *buf,
>  		goto out;
>  
>  	attr->set_buf[size] = '\0';
> -	val = simple_strtoll(attr->set_buf, NULL, 0);
> +	ret = kstrtoull(attr->set_buf, 0, &val);
> +	if (ret)
> +		goto out;
>  	ret = attr->set(attr->data, val);
>  	if (ret == 0)
>  		ret = len; /* on success, claim we got the whole input */

kstrtoull() takes an `unsigned long long *', but `val' is a u64.

I think this probably works OK on all architectures (ie, no 64-bit
architectures are using `unsigned long' for u64).  But perhaps `val'
should have type `unsigned long long'?