[PATCH] mtdchar: fix overflows in adjustment of `count`

[boris.brezillon@xxxxxxxxxxx (Boris Brezillon)]

On Sat,  7 Jul 2018 05:37:22 +0200
Jann Horn <jannh at google.com> wrote:

> The first checks in mtdchar_read() and mtdchar_write() attempt to limit
> `count` such that `*ppos + count <= mtd->size`. However, they ignore the
> possibility of `*ppos > mtd->size`, allowing the calculation of `count` to
> wrap around. `mtdchar_lseek()` prevents seeking beyond mtd->size, but the
> pread/pwrite syscalls bypass this.
> 
> I haven't found any codepath on which this actually causes dangerous
> behavior, but it seems like a sensible change anyway.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Jann Horn <jannh at google.com>
> ---
>  drivers/mtd/mtdchar.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/mtd/mtdchar.c b/drivers/mtd/mtdchar.c
> index cd67c85cc87d..02389528f622 100644
> --- a/drivers/mtd/mtdchar.c
> +++ b/drivers/mtd/mtdchar.c
> @@ -160,8 +160,12 @@ static ssize_t mtdchar_read(struct file *file, char __user *buf, size_t count,
>  
>  	pr_debug("MTD_read\n");
>  
> -	if (*ppos + count > mtd->size)
> -		count = mtd->size - *ppos;
> +	if (*ppos + count > mtd->size) {
> +		if (*ppos < mtd->size)
> +			count = mtd->size - *ppos;
> +		else
> +			count = 0;
> +	}

Hm, shouldn't we return -ERANGE or -EINVAL if *ppos >= mtd->size?

>  
>  	if (!count)
>  		return 0;
> @@ -246,7 +250,7 @@ static ssize_t mtdchar_write(struct file *file, const char __user *buf, size_t c
>  
>  	pr_debug("MTD_write\n");
>  
> -	if (*ppos == mtd->size)
> +	if (*ppos >= mtd->size)
>  		return -ENOSPC;
>  
>  	if (*ppos + count > mtd->size)