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Date:   Tue, 3 May 2022 22:31:05 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Rich Felker <dalias@...ifal.cx>,
        Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        alsa-devel@...a-project.org, Al Viro <viro@...iv.linux.org.uk>,
        Andrew Gabbasov <andrew_gabbasov@...tor.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Gross <agross@...nel.org>,
        Andy Lavr <andy.lavr@...il.com>,
        Arend van Spriel <aspriel@...il.com>,
        Baowen Zheng <baowen.zheng@...igine.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Bradley Grove <linuxdrivers@...otech.com>,
        brcm80211-dev-list.pdl@...adcom.com,
        Christian Brauner <brauner@...nel.org>,
        Christian Göttsche <cgzones@...glemail.com>,
        Christian Lamparter <chunkeey@...glemail.com>,
        Chris Zankel <chris@...kel.net>,
        Cong Wang <cong.wang@...edance.com>,
        Daniel Axtens <dja@...ens.net>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Dan Williams <dan.j.williams@...el.com>,
        David Gow <davidgow@...gle.com>,
        David Howells <dhowells@...hat.com>,
        Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
        devicetree@...r.kernel.org, Dexuan Cui <decui@...rosoft.com>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        Eli Cohen <elic@...dia.com>,
        Eric Paris <eparis@...isplace.org>,
        Eugeniu Rosca <erosca@...adit-jv.com>,
        Felipe Balbi <balbi@...nel.org>,
        Francis Laniel <laniel_francis@...vacyrequired.com>,
        Frank Rowand <frowand.list@...il.com>,
        Franky Lin <franky.lin@...adcom.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Gregory Greenman <gregory.greenman@...el.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Hante Meuleman <hante.meuleman@...adcom.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Hulk Robot <hulkci@...wei.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        James Morris <jmorris@...ei.org>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Jaroslav Kysela <perex@...ex.cz>,
        Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
        Johan Hedberg <johan.hedberg@...il.com>,
        Johannes Berg <johannes.berg@...el.com>,
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        "K. Y. Srinivasan" <kys@...rosoft.com>,
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        Lee Jones <lee.jones@...aro.org>,
        Leon Romanovsky <leon@...nel.org>,
        Liam Girdwood <lgirdwood@...il.com>,
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        linux-xtensa@...ux-xtensa.org, llvm@...ts.linux.dev,
        Loic Poulain <loic.poulain@...aro.org>,
        Louis Peens <louis.peens@...igine.com>,
        Luca Coelho <luciano.coelho@...el.com>,
        Luiz Augusto von Dentz <luiz.dentz@...il.com>,
        Marc Dionne <marc.dionne@...istor.com>,
        Marcel Holtmann <marcel@...tmann.org>,
        Mark Brown <broonie@...nel.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Max Filippov <jcmvbkbc@...il.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Muchun Song <songmuchun@...edance.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nuno Sá <nuno.sa@...log.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Paul Moore <paul@...l-moore.com>,
        Rob Herring <robh+dt@...nel.org>,
        Russell King <linux@...linux.org.uk>, selinux@...r.kernel.org,
        "Serge E. Hallyn" <serge@...lyn.com>,
        SHA-cyfmac-dev-list@...ineon.com,
        Simon Horman <simon.horman@...igine.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Stefan Richter <stefanr@...6.in-berlin.de>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Tadeusz Struk <tadeusz.struk@...aro.org>,
        Takashi Iwai <tiwai@...e.com>, Tom Rix <trix@...hat.com>,
        Udipto Goswami <quic_ugoswami@...cinc.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        wcn36xx@...ts.infradead.org, Wei Liu <wei.liu@...nel.org>,
        xen-devel@...ts.xenproject.org,
        Xiu Jianfeng <xiujianfeng@...wei.com>,
        Yang Yingliang <yangyingliang@...wei.com>
Subject: Re: [PATCH 01/32] netlink: Avoid memcpy() across flexible array
 boundary

On Tue, May 03, 2022 at 06:44:10PM -0700, Kees Cook wrote:
> In preparation for run-time memcpy() bounds checking, split the nlmsg
> copying for error messages (which crosses a previous unspecified flexible
> array boundary) in half. Avoids the future run-time warning:
> 
> memcpy: detected field-spanning write (size 32) of single field "&errmsg->msg" (size 16)
> 
> Creates an explicit flexible array at the end of nlmsghdr for the payload,
> named "nlmsg_payload". There is no impact on UAPI; the sizeof(struct
> nlmsghdr) does not change, but now the compiler can better reason about
> where things are being copied.
> 
> Fixed-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> Link: https://lore.kernel.org/lkml/d7251d92-150b-5346-6237-52afc154bb00@rasmusvillemoes.dk
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Jakub Kicinski <kuba@...nel.org>
> Cc: Rich Felker <dalias@...ifal.cx>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: netdev@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  include/uapi/linux/netlink.h | 1 +
>  net/netlink/af_netlink.c     | 5 ++++-
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
> index 855dffb4c1c3..47f9342d51bc 100644
> --- a/include/uapi/linux/netlink.h
> +++ b/include/uapi/linux/netlink.h
> @@ -47,6 +47,7 @@ struct nlmsghdr {
>  	__u16		nlmsg_flags;	/* Additional flags */
>  	__u32		nlmsg_seq;	/* Sequence number */
>  	__u32		nlmsg_pid;	/* Sending process port ID */
> +	__u8		nlmsg_payload[];/* Contents of message */
>  };
>  
>  /* Flags values */
> diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
> index 1b5a9c2e1c29..09346aee1022 100644
> --- a/net/netlink/af_netlink.c
> +++ b/net/netlink/af_netlink.c
> @@ -2445,7 +2445,10 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
>  			  NLMSG_ERROR, payload, flags);
>  	errmsg = nlmsg_data(rep);
>  	errmsg->error = err;
> -	memcpy(&errmsg->msg, nlh, payload > sizeof(*errmsg) ? nlh->nlmsg_len : sizeof(*nlh));
> +	errmsg->msg = *nlh;
> +	if (payload > sizeof(*errmsg))
> +		memcpy(errmsg->msg.nlmsg_payload, nlh->nlmsg_payload,
> +		       nlh->nlmsg_len - sizeof(*nlh));

They have nlmsg_len()[1] for the length of the payload without the header:

/**
 * nlmsg_len - length of message payload
 * @nlh: netlink message header
 */
static inline int nlmsg_len(const struct nlmsghdr *nlh)
{
	return nlh->nlmsg_len - NLMSG_HDRLEN;
}

(would that function use some sanitization, though? what if nlmsg_len is
somehow manipulated to be less than NLMSG_HDRLEN?...)

Also, it seems there is at least one more instance of this same issue:

diff --git a/net/netfilter/ipset/ip_set_core.c b/net/netfilter/ipset/ip_set_core.c
index 16ae92054baa..d06184b94af5 100644
--- a/net/netfilter/ipset/ip_set_core.c
+++ b/net/netfilter/ipset/ip_set_core.c
@@ -1723,7 +1723,8 @@ call_ad(struct net *net, struct sock *ctnl, struct sk_buff *skb,
                                  nlh->nlmsg_seq, NLMSG_ERROR, payload, 0);
                errmsg = nlmsg_data(rep);
                errmsg->error = ret;
-               memcpy(&errmsg->msg, nlh, nlh->nlmsg_len);
+               errmsg->msg = *nlh;
+               memcpy(errmsg->msg.nlmsg_payload, nlh->nlmsg_payload, nlmsg_len(nlh));
                cmdattr = (void *)&errmsg->msg + min_len;

                ret = nla_parse(cda, IPSET_ATTR_CMD_MAX, cmdattr,

--
Gustavo

[1] https://elixir.bootlin.com/linux/v5.18-rc5/source/include/net/netlink.h#L577

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