Subject: [PATCH RFC net-next 0/5] rtnetlink: Add support for rigid checking of data in dump request

From: [email-masked]
Date: Fri, 28 Sep 2018 08:44:57 -0700

From: David Ahern <[email-masked]>

There are many use cases where a user wants to influence what is
returned in a dump for some rtnetlink command: one is wanting data
for a different namespace than the one the request is received and
another is limiting the amount of data returned in the dump to a
specific set of interest to userspace, reducing the cpu overhead of
both kernel and userspace. Unfortunately, the kernel has historically
not been strict with checking for the proper header or checking the
values passed in the header. This lenient implementation has allowed
iproute2 and other packages to pass any struct or data in the dump
request as long as the family is the first byte. For example, ifinfomsg
struct is used by iproute2 for all generic dump requests - links,
addresses, routes and rules when it is really only valid for link
requests.

There is 1 is example where the kernel deals with the wrong struct: link
dumps after VF support was added. Older iproute2 was sending rtgenmsg as
the header instead of ifinfomsg so a patch was added to try and detect
old userspace vs new:
e5eca6d41f53 ("rtnetlink: fix userspace API breakage for iproute2 < v3.9.0")

The latest example is Christian's patch set wanting to return addresses for
a target namespace. It guesses the header struct is an ifaddrmsg and if it
guesses wrong a netlink warning is generated in the kernel log on every
address dump which is unacceptable.

Another example where the kernel is a bit lenient is route dumps: iproute2
can send either a request with either ifinfomsg or a rtmsg as the header
struct, yet the kernel always treats the header as an rtmsg (see
inet_dump_fib and rtm_flags check).

How to resolve the problem of not breaking old userspace yet be able to
move forward with new features such as kernel side filtering which are
crucial for efficient operation at high scale?

This patch set addresses the problem by adding a new netlink flag,
NLM_F_DUMP_PROPER_HDR, that userspace can set to say "I have a clue, and
I am sending the right header struct" and that the struct fields and any
attributes after it should be used for filtering the data returned in the
dump.

Kernel side, the dump handlers are updated to check every field in the
header struct and all attributes passed. Only ones where filtering is
implemented are allowed to be set. Any other values cause the dump to fail
with EINVAL. If the new flag is honored by the kernel and the dump contents
adjusted by any data passed in the request, the dump handler sets the
NLM_F_DUMP_FILTERED flag in the netlink message header.

This is an RFC set with the address handlers updated. If the approach is
acceptable, then I will do the same to the other rtnetlink dump handlers.

David Ahern (5):
  net/netlink: Pass extack to dump callbacks
  net/ipv6: Refactor address dump to push inet6_fill_args to
    in6_dump_addrs
  netlink: introduce NLM_F_DUMP_PROPER_HDR flag
  net/ipv4: Update inet_dump_ifaddr to support NLM_F_DUMP_PROPER_HDR
  net/ipv6: Update inet6_dump_addr to support NLM_F_DUMP_PROPER_HDR

 include/linux/netlink.h      |   2 +
 include/uapi/linux/netlink.h |   1 +
 net/core/rtnetlink.c         |   1 +
 net/ipv4/devinet.c           |  52 +++++++++++++++++-----
 net/ipv6/addrconf.c          | 101 +++++++++++++++++++++++++++++--------------
 net/netlink/af_netlink.c     |   1 +
 6 files changed, 114 insertions(+), 44 deletions(-)

-- 
2.11.0