From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [RFC] tcp md5 use of alloc_percpu
Date: Wed, 22 Oct 2014 17:53:24 -0400 (EDT)
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From: Crestez Dan Leonard <cdleonard@gmail.com>
Date: Wed, 22 Oct 2014 21:55:46 +0300

>  static int tcp_v4_md5_hash_pseudoheader(struct tcp_md5sig_pool *hp,
>  					__be32 daddr, __be32 saddr, int nbytes)
>  {
> -	struct tcp4_pseudohdr *bp;
> +	struct tcp4_pseudohdr bp;
...
> +	sg_init_one(&sg, &bp, sizeof(bp));
> +	return crypto_hash_update(&hp->md5_desc, &sg, sizeof(bp));

As others have mentioned, you cannot do this.

On some architectures the kernel stack comes from vmalloc()
memory too.