ipv4: stop PMTU walk when nexthop group shrinks

Commit 7d3f3b4367f3 ("net: ipv4: Cache pmtu for all packet paths if
multipath enabled") made __ip_rt_update_pmtu() update every path.

For nexthop objects, fib_info_num_path() and fib_info_nhc()
independently load nh->nh_grp.  RCU protects each group's lifetime but
does not make the loads observe the same group.  If replacement shrinks
the group after the loop accepts an index, fib_info_nhc() returns NULL
and update_or_create_fnhe() dereferences it.

A deterministic probe only widened the existing window between the real
operations.  On v7.2, an RTM_NEWNEXTHOP replacement published a
one-member group after the PMTU reader accepted index 1 from a two-member
group, producing:

  KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
  RIP: update_or_create_fnhe+0x45/0x15b0

Stop when the indexed path is absent.  Groups are dense, so no later
index exists in that snapshot.  The same real-writer window completed
114,423 PMTU iterations without an oops with this check.  A reproducer
is available on request.

Replacement requires CAP_NET_ADMIN in the network namespace.  Where
unprivileged user namespaces are permitted, a local user can obtain it
in a new user and network namespace.

Fixes: 7d3f3b4367f3 ("net: ipv4: Cache pmtu for all packet paths if multipath enabled")
Cc: stable@vger.kernel.org
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/4fb87ed994dfc067178fc845cdd8bc9b5446b039.1790915964.git.4ncienth@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions