Re: [PATCH net v2 1/2] ipv4: Fix route lookups when handling ICMP redirects and PMTU updates

[David Ahern <dsahern@kernel.org>]

On 3/17/22 6:45 AM, Guillaume Nault wrote:
> The PMTU update and ICMP redirect helper functions initialise their fl4
> variable with either __build_flow_key() or build_sk_flow_key(). These
> initialisation functions always set ->flowi4_scope with
> RT_SCOPE_UNIVERSE and might set the ECN bits of ->flowi4_tos. This is
> not a problem when the route lookup is later done via
> ip_route_output_key_hash(), which properly clears the ECN bits from
> ->flowi4_tos and initialises ->flowi4_scope based on the RTO_ONLINK
> flag. However, some helpers call fib_lookup() directly, without
> sanitising the tos and scope fields, so the route lookup can fail and,
> as a result, the ICMP redirect or PMTU update aren't taken into
> account.
> 
> Fix this by extracting the ->flowi4_tos and ->flowi4_scope sanitisation
> code into ip_rt_fix_tos(), then use this function in handlers that call
> fib_lookup() directly.
> 
> Note 1: We can't sanitise ->flowi4_tos and ->flowi4_scope in a central
> place (like __build_flow_key() or flowi4_init_output()), because
> ip_route_output_key_hash() expects non-sanitised values. When called
> with sanitised values, it can erroneously overwrite RT_SCOPE_LINK with
> RT_SCOPE_UNIVERSE in ->flowi4_scope. Therefore we have to be careful to
> sanitise the values only for those paths that don't call
> ip_route_output_key_hash().
> 
> Note 2: The problem is mostly about sanitising ->flowi4_tos. Having
> ->flowi4_scope initialised with RT_SCOPE_UNIVERSE instead of
> RT_SCOPE_LINK probably wasn't really a problem: sockets with the
> SOCK_LOCALROUTE flag set (those that'd result in RTO_ONLINK being set)
> normally shouldn't receive ICMP redirects or PMTU updates.
> 
> Fixes: 4895c771c7f0 ("ipv4: Add FIB nexthop exceptions.")
> Signed-off-by: Guillaume Nault <gnault@redhat.com>
> ---
>  net/ipv4/route.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 

Reviewed-by: David Ahern <dsahern@kernel.org>