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Message-ID: <CAF=yD-JbBO2rRjUKFSNra+4TTQ7J6QGiXLPzL_Mc6TLJTFUFfA@mail.gmail.com> Date: Tue, 23 Apr 2019 13:04:13 -0400 From: Willem de Bruijn <willemdebruijn.kernel@...il.com> To: David Laight <David.Laight@...lab.com> Cc: "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "davem@...emloft.net" <davem@...emloft.net>, "idosch@...sch.org" <idosch@...sch.org>, Willem de Bruijn <willemb@...gle.com> Subject: Re: [PATCH net] packet: validate address length if non-zero On Tue, Apr 23, 2019 at 11:52 AM David Laight <David.Laight@...lab.com> wrote: > > From: Willem de Bruijn > > Sent: 23 April 2019 16:08 > > On Tue, Apr 23, 2019 at 5:59 AM David Laight <David.Laight@...lab.com> wrote: > > > > > > From: Willem de Bruijn > > > > Sent: 22 December 2018 21:54 > > > > Validate packet socket address length if a length is given. Zero > > > > length is equivalent to not setting an address. > > > > > > > > Fixes: 99137b7888f4 ("packet: validate address length") > > > > Reported-by: Ido Schimmel <idosch@...sch.org> > > > > Signed-off-by: Willem de Bruijn <willemb@...gle.com> > > > > --- > > > > net/packet/af_packet.c | 4 ++-- > > > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > > > > > diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c > > > > index 5dda263b4a0a..eedacdebcd4c 100644 > > > > --- a/net/packet/af_packet.c > > > > +++ b/net/packet/af_packet.c > > > > @@ -2625,7 +2625,7 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg) > > > > sll_addr))) > > > > goto out; > > > > proto = saddr->sll_protocol; > > > > - addr = saddr->sll_addr; > > > > + addr = saddr->sll_halen ? saddr->sll_addr : NULL; > > > > dev = dev_get_by_index(sock_net(&po->sk), saddr->sll_ifindex); > > > > if (addr && dev && saddr->sll_halen < dev->addr_len) > > > > goto out; > > > > @@ -2825,7 +2825,7 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len) > > > > if (msg->msg_namelen < (saddr->sll_halen + offsetof(struct sockaddr_ll, sll_addr))) > > > > goto out; > > > > proto = saddr->sll_protocol; > > > > - addr = saddr->sll_addr; > > > > + addr = saddr->sll_halen ? saddr->sll_addr : NULL; > > > > dev = dev_get_by_index(sock_net(sk), saddr->sll_ifindex); > > > > if (addr && dev && saddr->sll_halen < dev->addr_len) > > > > goto out; > > > > -- > > > > 2.20.1.415.g653613c723-goog > > > > > > We've just discovered the combination of this patch and the one it 'fixes' > > > breaks some of our userspace code. > > > > > > Prior to these changes it didn't matter if code using AF_PACKET to > > > send ethernet frames on a specific 'ethertype' failed to set sll_addr. > > > Everything assumed it would be 6 - and the packets were sent. > > > > > > With both changes you get a -EINVAL return from somewhere. > > > I can fix our code, but I doubt it is the only code affected. > > > Other people are likely to have copied the same example. > > > > Thanks for the report. > > > > Usage trumps correctness. But this seems to be a case of damned if you > > do, damned if you don't. > > > > Syzbot found a real use case of reading beyond the end of > > msg->msg_namelen, since that is checked against > > > > if (msg->msg_namelen < (saddr->sll_halen + > > offsetof(struct sockaddr_ll, sll_addr))) > > > > Just assuming that address length is dev->addr_len allows an > > ns_capable root to build link layer packets with address set to > > uninitialized data. > > > > Ethernet is not the most problematic link layer. Indeed, since > > ETH_ALEN < sizeof(sll_addr), the previous check > > > > if (msg->msg_namelen < sizeof(struct sockaddr_ll)) > > > > Will be sufficient in this case. The syzbot report was on a device of > > type ip6gre, with addr_len sizeof(struct in6_addr). > > > > So I can refine to only perform the check on protocols with addr_len > > >= sizeof(sll_addr), excluding Ethernet. > > Maybe something like: > addr = saddr->sll_addr; > dev = dev_get_by_index(sock_net(sk), saddr->sll_ifindex); > if (dev && msg->msg_namelen < (dev->addr_len > + offsetof(struct sockaddr_ll, > sll_addr))) > /* Don't read address from beyond the end of the buffer */ > goto out; > > So it just checks that all of the address is in the buffer passed > from the user. Yes. sll_halen is never used outside this block. Testing that against dev->addr_len just adds needless a level of indirection. We only care that code that assumes the address is dev->addr_len won't read beyond the end of msg->namelen. So this looks great to me (aside from goto out_unlock). Thanks. > Although I'd write those tests in a different order: > if (dev && offsetof(struct sockaddr_ll, sll_addr) + dev->addr_len > msg->msg_namelen) > goto out; Since there already is a test for msg->msg_namelen < sizeof(struct sockaddr_ll) just before, I'd keep the same order. Do you want to send a fix, or shall I?
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