Re: [PATCH v3 2/2] mm/memfd: Use strncpy_from_user() to read memfd name

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Thu, Jan 09, 2025 at 10:59:05AM -0800, Isaac J. Manjarres wrote:
> The existing logic uses strnlen_user() to calculate the length of the
> memfd name from userspace and then copies the string into a buffer using
> copy_from_user(). This is error-prone, as the string length
> could have changed between the time when it was calculated and when the
> string was copied. The existing logic handles this by ensuring that the
> last byte in the buffer is the terminating zero.
>
> This handling is contrived and can better be handled by using
> strncpy_from_user(), which gets the length of the string and copies
> it in one shot. Therefore, simplify the logic for copying the memfd
> name by using strncpy_from_user().
>
> No functional change.
>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>

LGTM,

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  mm/memfd.c | 20 ++++++--------------
>  1 file changed, 6 insertions(+), 14 deletions(-)
>
> diff --git a/mm/memfd.c b/mm/memfd.c
> index bf0c2d97b940..5b7c5892ba64 100644
> --- a/mm/memfd.c
> +++ b/mm/memfd.c
> @@ -396,26 +396,18 @@ static char *alloc_name(const char __user *uname)
>  	char *name;
>  	long len;
>
> -	/* length includes terminating zero */
> -	len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
> -	if (len <= 0)
> -		return ERR_PTR(-EFAULT);
> -	if (len > MFD_NAME_MAX_LEN + 1)
> -		return ERR_PTR(-EINVAL);
> -
> -	name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL);
> +	name = kmalloc(NAME_MAX + 1, GFP_KERNEL);
>  	if (!name)
>  		return ERR_PTR(-ENOMEM);
>
>  	strcpy(name, MFD_NAME_PREFIX);
> -	if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) {
> +	/* returned length does not include terminating zero */
> +	len = strncpy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, MFD_NAME_MAX_LEN + 1);
> +	if (len < 0) {
>  		error = -EFAULT;
>  		goto err_name;
> -	}
> -
> -	/* terminating-zero may have changed after strnlen_user() returned */
> -	if (name[len + MFD_NAME_PREFIX_LEN - 1]) {
> -		error = -EFAULT;
> +	} else if (len > MFD_NAME_MAX_LEN) {
> +		error = -EINVAL;
>  		goto err_name;
>  	}
>
> --
> 2.47.1.613.gc27f4b7a9f-goog
>