Re: [PATCHv2] Fix range checks in kernfs_get_target_path

[Greg Kroah-Hartman]

On Sat, Jul 07, 2018 at 09:41:03AM +0000, Bernd Edlinger wrote:
> The strncpy causes a warning [-Wstringop-truncation] here,
> which indicates that it never appends a NUL byte to the path.
> The NUL byte is only there because the buffer is allocated
> with kzalloc(PAGE_SIZE, GFP_KERNEL), but since the range-check
> is also off-by-one, and PAGE_SIZE==PATH_MAX the returned string
> will not be zero-terminated if it is exactly PATH_MAX characters.
> Furthermore also the initial loop may theoretically exceed PATH_MAX
> and cause a fault.
> 
> Signed-off-by: Bernd Edlinger <bernd.edlinger@xxxxxxxxxx>
> ---
>   fs/kernfs/symlink.c | 10 +++++++---
>   1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/kernfs/symlink.c b/fs/kernfs/symlink.c
> index 08ccabd..c8b7d44a 100644
> --- a/fs/kernfs/symlink.c
> +++ b/fs/kernfs/symlink.c
> @@ -63,7 +63,10 @@ static int kernfs_get_target_path(struct kernfs_node
>   		if (base == kn)
>   			break;
> 
> -		strcpy(s, "../");
> +		if ((s - path) + 3 >= PATH_MAX)
> +			return -ENAMETOOLONG;
> +
> +		memcpy(s, "../", 3);
>   		s += 3;
>   		base = base->parent;
>   	}
> @@ -79,16 +82,17 @@ static int kernfs_get_target_path(struct kernfs_node
>   	if (len < 2)
>   		return -EINVAL;
>   	len--;
> -	if ((s - path) + len > PATH_MAX)
> +	if ((s - path) + len >= PATH_MAX)
>   		return -ENAMETOOLONG;
> 
>   	/* reverse fillup of target string from target to base */
>   	kn = target;
> +	s[len] = '\0';
>   	while (kn->parent && kn != base) {
>   		int slen = strlen(kn->name);
> 
>   		len -= slen;
> -		strncpy(s + len, kn->name, slen);
> +		memcpy(s + len, kn->name, slen);
>   		if (len)
>   			s[--len] = '/';
> 

This last memcpy replacement has already been applied to my tree, from a
patch from soeone else, so are you sure all of the other changes are
also really needed?  Why the extra \0 termination of a string that is
already terminated?

And why is the first memcpy replacement needed?  gcc doesn't say
anything about that, does it?

thanks,

greg k-h