Re: [PATCH v2] libfs: fix error cast of negative value in simple_attr_write()

[Al Viro]

On Fri, Nov 13, 2020 at 05:56:09PM +0800, Yicong Yang 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.
> 
> Fixes: f7b88631a897 ("fs/libfs.c: fix simple_attr_write() on 32bit machines")
> Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
> ---
> Change since v1:
> - address the compile warning for non-64 bit platform
> 
>  fs/libfs.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/libfs.c b/fs/libfs.c
> index fc34361..3a0d99c 100644
> --- 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, (unsigned long long *)&val);
> +	if (ret)
> +		goto out;
>  	ret = attr->set(attr->data, val);
>  	if (ret == 0)
>  		ret = len; /* on success, claim we got the whole input */

Make val unsigned long long instead.