Re: [ipsec-next,v6,12/14] xfrm: add XFRM_MSG_MIGRATE_STATE for single SA migration

[Simon Horman <horms@kernel.org>]

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---
xfrm: add XFRM_MSG_MIGRATE_STATE for single SA migration

This patch adds a new netlink method to migrate a single xfrm_state.
Unlike the existing migration mechanism (SA + policy), this supports
migrating only the SA and allows changing the reqid.

> diff --git a/include/net/xfrm.h b/include/net/xfrm.h
> index 4b29ab92c2a7..ae2a1cf2311a 100644
> --- a/include/net/xfrm.h
> +++ b/include/net/xfrm.h
> @@ -683,11 +683,17 @@ struct xfrm_migrate {
>  	xfrm_address_t		new_daddr;
>  	xfrm_address_t		new_saddr;
>  	struct xfrm_encap_tmpl *encap;
> -	struct xfrm_user_offload *xuo;
> +	struct xfrm_user_offload xuo;

[ ... ]

> diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
> index 2f23ca5b62a9..de8b038fc08c 100644
> --- a/net/xfrm/xfrm_policy.c
> +++ b/net/xfrm/xfrm_policy.c
> @@ -4642,6 +4642,19 @@ static int xfrm_migrate_check(const struct xfrm_migrate *m, int num_migrate,
>  	return 0;
>  }
>
> +/* Fill migrate fields that are invariant in XFRM_MSG_MIGRATE: inherited
> + * from the existing SA unchanged.
> + */
> +static void xfrm_migrate_copy_old(struct xfrm_migrate *mp,
> +				  const struct xfrm_state *x)
> +{
> +	mp->smark                  = x->props.smark;
> +	mp->new_reqid              = x->props.reqid;
> +	mp->nat_keepalive_interval = x->nat_keepalive_interval;
> +	mp->mapping_maxage         = x->mapping_maxage;
> +	mp->new_mark               = x->mark;
> +}
> +
>  int xfrm_migrate(const struct xfrm_selector *sel, u8 dir, u8 type,
>  		 struct xfrm_migrate *m, int num_migrate,
>  		 struct xfrm_kmaddress *k, struct net *net,
> @@ -4680,7 +4693,10 @@ int xfrm_migrate(const struct xfrm_selector *sel, u8 dir, u8 type,
>  			x_cur[nx_cur] = x;
>  			nx_cur++;
>  			mp->encap = encap;
> -			mp->xuo = xuo;
> +			if (xuo)
> +				mp->xuo = *xuo;
                                     ^^^^
>  			xfrm_migrate_copy_old(mp, x);
>

What happens to mp->xuo when xuo is NULL?

The xfrm_migrate structure changed its xuo field from a pointer
(struct xfrm_user_offload *xuo) to an embedded value
(struct xfrm_user_offload xuo).

In xfrm_migrate(), when the xuo parameter is NULL (PF_KEY path via
pfkey_migrate, or XFRM_MSG_MIGRATE without XFRMA_OFFLOAD_DEV), the
'if (xuo) mp->xuo = *xuo' branch is skipped, leaving mp->xuo with
uninitialized stack data.

Both callers declare 'struct xfrm_migrate m[XFRM_MAX_DEPTH]' on the
stack without zeroing:

net/key/af_key.c:pfkey_migrate() {
    struct xfrm_migrate m[XFRM_MAX_DEPTH];
    ...
    err = xfrm_migrate(&sel, dir, XFRM_POLICY_TYPE_MAIN, m, i,
                       k.local && k.remote ? &k : NULL, net,
                       NULL, 0, NULL);
}

net/xfrm/xfrm_user.c:xfrm_do_migrate() {
    struct xfrm_migrate m[XFRM_MAX_DEPTH];
    ...
    err = xfrm_migrate(&sel, dir, type, m, n, k, net, encap, xuo, extack);
}

The uninitialized mp->xuo.ifindex is later read in
xfrm_state_migrate_create() and xfrm_state_migrate_install():

net/xfrm/xfrm_state.c:xfrm_state_migrate_create() {
    ...
    /* configure the hardware if offload is requested */
    if (m->xuo.ifindex && xfrm_dev_state_add(net, xc, &m->xuo, extack))
        goto error;
}

net/xfrm/xfrm_state.c:xfrm_state_migrate_install() {
    ...
    if (xfrm_state_add(xc) < 0) {
        NL_SET_ERR_MSG(extack, "Failed to add migrated state");
        if (m->xuo.ifindex)
            xfrm_dev_state_delete(xc);
        ...
    }
}

If the stack garbage contains a non-zero ifindex value,
xfrm_dev_state_add() will be called with random ifindex and flags,
potentially causing spurious hardware offload attempts or migration
failures.

Previously, 'mp->xuo = xuo' unconditionally set the pointer to NULL in
these paths, which was then properly checked as 'if (m->xuo)'.

Should mp->xuo be zero-initialized when xuo is NULL, either by adding
'else memset(&mp->xuo, 0, sizeof(mp->xuo))' after the if-block, or by
zero-initializing the m[] arrays in the callers?