Re: [PATCHv2 net] icmp: fix icmp_ext_echo_iio parsing in icmp_build_probe

On Thu, Oct 14, 2021 at 12:43 PM Eric Dumazet [off-list ref] wrote:

On 10/13/21 9:24 PM, Xin Long wrote:

quoted

In icmp_build_probe(), the icmp_ext_echo_iio parsing should be done
step by step and skb_header_pointer() return value should always be
checked, this patch fixes 3 places in there:

  - On case ICMP_EXT_ECHO_CTYPE_NAME, it should only copy ident.name
    from skb by skb_header_pointer(), its len is ident_len. Besides,
    the return value of skb_header_pointer() should always be checked.

  - On case ICMP_EXT_ECHO_CTYPE_INDEX, move ident_len check ahead of
    skb_header_pointer(), and also do the return value check for
    skb_header_pointer().

  - On case ICMP_EXT_ECHO_CTYPE_ADDR, before accessing iio->ident.addr.
    ctype3_hdr.addrlen, skb_header_pointer() should be called first,
    then check its return value and ident_len.
    On subcases ICMP_AFI_IP and ICMP_AFI_IP6, also do check for ident.
    addr.ctype3_hdr.addrlen and skb_header_pointer()'s return value.
    On subcase ICMP_AFI_IP, the len for skb_header_pointer() should be
    "sizeof(iio->extobj_hdr) + sizeof(iio->ident.addr.ctype3_hdr) +
    sizeof(struct in_addr)" or "ident_len".

v1->v2:
  - To make it more clear, call skb_header_pointer() once only for
    iio->indent's parsing as Jakub Suggested.

Fixes: d329ea5bd884 ("icmp: add response to RFC 8335 PROBE messages")
Reported-by: Dan Carpenter <redacted>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
---
 net/ipv4/icmp.c | 20 +++++++++-----------
 1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index 8b30cadff708..bccb2132a464 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c

@@ -1057,11 +1057,15 @@ bool icmp_build_probe(struct sk_buff *skb, struct icmphdr *icmphdr)
      if (ntohs(iio->extobj_hdr.length) <= sizeof(iio->extobj_hdr))
              goto send_mal_query;
      ident_len = ntohs(iio->extobj_hdr.length) - sizeof(iio->extobj_hdr);
+     iio = skb_header_pointer(skb, sizeof(_ext_hdr),
+                              sizeof(iio->extobj_hdr) + ident_len, &_iio);

??? How has this been tested ???

This actually is difficult to cook a non-linear skb to be tested.
In the testing, if it's a linear skb, I realized it won't use &_iio memory.
when the value was greater than skb's len, it returned NULL.
when the value was less than skb's len, it just used skb->data memory.

If you pass &_iio for last argument, then you _must_ use sizeof(__iio) (or smaller) too for third argument,
or risk stack overflow, in the case page frag bytes need to be copied into _iio

If the remote peer cooks a malicious packet so that ident_len is big like 1200,
then for sure the kernel will crash,
because sizeof(iio->extobj_hdr) + ident_len will be bigger than sizeof(_iio)

You're right, more check is needed before calling skb_header_pointer():

-       if (ntohs(iio->extobj_hdr.length) <= sizeof(iio->extobj_hdr))
+       if (ntohs(iio->extobj_hdr.length) <= sizeof(iio->extobj_hdr) ||
+           ntohs(iio->extobj_hdr.length) > sizeof(_iio))
                goto send_mal_query;
        ident_len = ntohs(iio->extobj_hdr.length) - sizeof(iio->extobj_hdr);
        iio = skb_header_pointer(skb, sizeof(_ext_hdr),

Thanks.

quoted

+     if (!iio)
+             goto send_mal_query;
+
      status = 0;
      dev = NULL;
      switch (iio->extobj_hdr.class_type) {