* [PATCH] netlink: netlink_sendmsg: memset unused tail bytes in skb @ 2021-05-09 12:18 Phillip Potter 2021-05-09 13:10 ` Florian Westphal 0 siblings, 1 reply; 3+ messages in thread From: Phillip Potter @ 2021-05-09 12:18 UTC (permalink / raw) To: davem Cc: kuba, yhs, ast, johannes.berg, rdunlap, 0x7f454c46, yangyingliang, netdev, linux-kernel When allocating the skb within netlink_sendmsg, with certain supplied len arguments, extra bytes are allocated at the end of the data buffer, due to SKB_DATA_ALIGN giving a larger size within __alloc_skb for alignment reasons. This means that after using skb_put with the same len value and then copying data into the skb, the skb tail area is non-zero in size and contains uninitialised bytes. Wiping this area (if it exists) fixes a KMSAN-found uninit-value bug reported by syzbot at: https://syzkaller.appspot.com/bug?id=3e63bcec536b7136b54c72e06adeb87dc6519f69 Reported-by: syzbot+a73d24a22eeeebe5f244@syzkaller.appspotmail.com Signed-off-by: Phillip Potter <phil@philpotter.co.uk> --- net/netlink/af_netlink.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c index 3a62f97acf39..e54321b63f98 100644 --- a/net/netlink/af_netlink.c +++ b/net/netlink/af_netlink.c @@ -1914,6 +1914,9 @@ static int netlink_sendmsg(struct socket *sock, struct msghdr *msg, size_t len) goto out; } + if (skb->end - skb->tail) + memset(skb_tail_pointer(skb), 0, skb->end - skb->tail); + err = security_netlink_send(sk, skb); if (err) { kfree_skb(skb); -- 2.30.2 ^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] netlink: netlink_sendmsg: memset unused tail bytes in skb 2021-05-09 12:18 [PATCH] netlink: netlink_sendmsg: memset unused tail bytes in skb Phillip Potter @ 2021-05-09 13:10 ` Florian Westphal 2021-05-11 22:17 ` Phillip Potter 0 siblings, 1 reply; 3+ messages in thread From: Florian Westphal @ 2021-05-09 13:10 UTC (permalink / raw) To: Phillip Potter Cc: davem, kuba, yhs, ast, johannes.berg, rdunlap, 0x7f454c46, yangyingliang, netdev, linux-kernel Phillip Potter <phil@philpotter.co.uk> wrote: > When allocating the skb within netlink_sendmsg, with certain supplied > len arguments, extra bytes are allocated at the end of the data buffer, > due to SKB_DATA_ALIGN giving a larger size within __alloc_skb for > alignment reasons. This means that after using skb_put with the same > len value and then copying data into the skb, the skb tail area is > non-zero in size and contains uninitialised bytes. Wiping this area > (if it exists) fixes a KMSAN-found uninit-value bug reported by syzbot at: > https://syzkaller.appspot.com/bug?id=3e63bcec536b7136b54c72e06adeb87dc6519f69 This patch papers over the real bug. Please fix TIPC instead. Incomplete patch as a starting point: diff --git a/net/tipc/node.c b/net/tipc/node.c --- a/net/tipc/node.c +++ b/net/tipc/node.c @@ -2481,7 +2481,6 @@ int tipc_nl_node_get_link(struct sk_buff *skb, struct genl_info *info) struct net *net = genl_info_net(info); struct nlattr *attrs[TIPC_NLA_LINK_MAX + 1]; struct tipc_nl_msg msg; - char *name; int err; msg.portid = info->snd_portid; @@ -2499,13 +2498,11 @@ int tipc_nl_node_get_link(struct sk_buff *skb, struct genl_info *info) if (!attrs[TIPC_NLA_LINK_NAME]) return -EINVAL; - name = nla_data(attrs[TIPC_NLA_LINK_NAME]); - msg.skb = nlmsg_new(NLMSG_GOODSIZE, GFP_KERNEL); if (!msg.skb) return -ENOMEM; - if (strcmp(name, tipc_bclink_name) == 0) { + if (nla_strcmp(attrs[TIPC_NLA_LINK_NAME], tipc_bclink_name) == 0) { err = tipc_nl_add_bc_link(net, &msg, tipc_net(net)->bcl); if (err) goto err_free; You will also need to change tipc_node_find_by_name() to pass the nla attr. Alternatively TIPC_NLA_LINK_NAME policy can be changed: diff --git a/net/tipc/netlink.c b/net/tipc/netlink.c --- a/net/tipc/netlink.c +++ b/net/tipc/netlink.c @@ -88,7 +88,7 @@ const struct nla_policy tipc_nl_net_policy[TIPC_NLA_NET_MAX + 1] = { const struct nla_policy tipc_nl_link_policy[TIPC_NLA_LINK_MAX + 1] = { [TIPC_NLA_LINK_UNSPEC] = { .type = NLA_UNSPEC }, - [TIPC_NLA_LINK_NAME] = { .type = NLA_STRING, + [TIPC_NLA_LINK_NAME] = { .type = NLA_NUL_STRING, ... which makes it safe to treat the raw attribute payload as a c-string, but this might break existing userspace applications. Its probably a good idea to audit all NLA_STRING attributes in tipc for similar problems. ^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] netlink: netlink_sendmsg: memset unused tail bytes in skb 2021-05-09 13:10 ` Florian Westphal @ 2021-05-11 22:17 ` Phillip Potter 0 siblings, 0 replies; 3+ messages in thread From: Phillip Potter @ 2021-05-11 22:17 UTC (permalink / raw) To: Florian Westphal Cc: davem, kuba, yhs, ast, johannes.berg, rdunlap, 0x7f454c46, yangyingliang, netdev, linux-kernel On Sun, May 09, 2021 at 03:10:51PM +0200, Florian Westphal wrote: > Phillip Potter <phil@philpotter.co.uk> wrote: > > When allocating the skb within netlink_sendmsg, with certain supplied > > len arguments, extra bytes are allocated at the end of the data buffer, > > due to SKB_DATA_ALIGN giving a larger size within __alloc_skb for > > alignment reasons. This means that after using skb_put with the same > > len value and then copying data into the skb, the skb tail area is > > non-zero in size and contains uninitialised bytes. Wiping this area > > (if it exists) fixes a KMSAN-found uninit-value bug reported by syzbot at: > > https://syzkaller.appspot.com/bug?id=3e63bcec536b7136b54c72e06adeb87dc6519f69 > > This patch papers over the real bug. > > Please fix TIPC instead. > Incomplete patch as a starting point: > > diff --git a/net/tipc/node.c b/net/tipc/node.c > --- a/net/tipc/node.c > +++ b/net/tipc/node.c > @@ -2481,7 +2481,6 @@ int tipc_nl_node_get_link(struct sk_buff *skb, struct genl_info *info) > struct net *net = genl_info_net(info); > struct nlattr *attrs[TIPC_NLA_LINK_MAX + 1]; > struct tipc_nl_msg msg; > - char *name; > int err; > > msg.portid = info->snd_portid; > @@ -2499,13 +2498,11 @@ int tipc_nl_node_get_link(struct sk_buff *skb, struct genl_info *info) > if (!attrs[TIPC_NLA_LINK_NAME]) > return -EINVAL; > > - name = nla_data(attrs[TIPC_NLA_LINK_NAME]); > - > msg.skb = nlmsg_new(NLMSG_GOODSIZE, GFP_KERNEL); > if (!msg.skb) > return -ENOMEM; > > - if (strcmp(name, tipc_bclink_name) == 0) { > + if (nla_strcmp(attrs[TIPC_NLA_LINK_NAME], tipc_bclink_name) == 0) { > err = tipc_nl_add_bc_link(net, &msg, tipc_net(net)->bcl); > if (err) > goto err_free; > > > You will also need to change tipc_node_find_by_name() to pass the nla > attr. > > Alternatively TIPC_NLA_LINK_NAME policy can be changed: > > diff --git a/net/tipc/netlink.c b/net/tipc/netlink.c > --- a/net/tipc/netlink.c > +++ b/net/tipc/netlink.c > @@ -88,7 +88,7 @@ const struct nla_policy tipc_nl_net_policy[TIPC_NLA_NET_MAX + 1] = { > > const struct nla_policy tipc_nl_link_policy[TIPC_NLA_LINK_MAX + 1] = { > [TIPC_NLA_LINK_UNSPEC] = { .type = NLA_UNSPEC }, > - [TIPC_NLA_LINK_NAME] = { .type = NLA_STRING, > + [TIPC_NLA_LINK_NAME] = { .type = NLA_NUL_STRING, > > > ... which makes it safe to treat the raw attribute payload as a c-string, > but this might break existing userspace applications. > > Its probably a good idea to audit all NLA_STRING attributes in tipc for > similar problems. Dear Florian, Thank you for your feedback and code + suggestions, I will take a look at this over the next few days and then resubmit. Regards, Phil ^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2021-05-11 22:17 UTC | newest] Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2021-05-09 12:18 [PATCH] netlink: netlink_sendmsg: memset unused tail bytes in skb Phillip Potter 2021-05-09 13:10 ` Florian Westphal 2021-05-11 22:17 ` Phillip Potter
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