Re: [PATCH] net: compat: fix msg_controllen overflow in scm_detach_fds_compat()

[Daniel Borkmann]

On 06/04/2019 02:31 PM, Young Xiao wrote:
> There is a missing check between kmsg->msg_controllen and cmlen,
> which can possibly lead to overflow.
> 
> This bug is similar to vulnerability that was fixed in commit 6900317f5eff
> ("net, scm: fix PaX detected msg_controllen overflow in scm_detach_fds").

Back then I mentioned in commit 6900317f5eff:

    In case of MSG_CMSG_COMPAT (scm_detach_fds_compat()), I haven't seen an
    issue in my tests as alignment seems always on 4 byte boundary. Same
    should be in case of native 32 bit, where we end up with 4 byte boundaries
    as well.

Do you have an actual reproducer or is it based on code inspection?

> Signed-off-by: Young Xiao <92siuyang@xxxxxxxxx>
> ---
>  net/compat.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/compat.c b/net/compat.c
> index a031bd3..8e74dfb 100644
> --- a/net/compat.c
> +++ b/net/compat.c
> @@ -301,6 +301,8 @@ void scm_detach_fds_compat(struct msghdr *kmsg, struct scm_cookie *scm)
>  			err = put_user(cmlen, &cm->cmsg_len);
>  		if (!err) {
>  			cmlen = CMSG_COMPAT_SPACE(i * sizeof(int));
> +			if (kmsg->msg_controllen < cmlen)
> +				cmlen = kmsg->msg_controllen;
>  			kmsg->msg_control += cmlen;
>  			kmsg->msg_controllen -= cmlen;
>  		}
>