Re: [PATCH net] ipv4: fix race condition between route lookup and invalidation

[Ido Schimmel <idosch@idosch.org>]

On Wed, Oct 16, 2019 at 12:03:15PM -0700, Wei Wang wrote:
> Jesse and Ido reported the following race condition:
> <CPU A, t0> - Received packet A is forwarded and cached dst entry is
> taken from the nexthop ('nhc->nhc_rth_input'). Calls skb_dst_set()
> 
> <t1> - Given Jesse has busy routers ("ingesting full BGP routing tables
> from multiple ISPs"), route is added / deleted and rt_cache_flush() is
> called
> 
> <CPU B, t2> - Received packet B tries to use the same cached dst entry
> from t0, but rt_cache_valid() is no longer true and it is replaced in
> rt_cache_route() by the newer one. This calls dst_dev_put() on the
> original dst entry which assigns the blackhole netdev to 'dst->dev'
> 
> <CPU A, t3> - dst_input(skb) is called on packet A and it is dropped due
> to 'dst->dev' being the blackhole netdev
> 
> There are 2 issues in the v4 routing code:
> 1. A per-netns counter is used to do the validation of the route. That
> means whenever a route is changed in the netns, users of all routes in
> the netns needs to redo lookup. v6 has an implementation of only
> updating fn_sernum for routes that are affected.
> 2. When rt_cache_valid() returns false, rt_cache_route() is called to
> throw away the current cache, and create a new one. This seems
> unnecessary because as long as this route does not change, the route
> cache does not need to be recreated.
> 
> To fully solve the above 2 issues, it probably needs quite some code
> changes and requires careful testing, and does not suite for net branch.
> 
> So this patch only tries to add the deleted cached rt into the uncached
> list, so user could still be able to use it to receive packets until
> it's done.
> 
> Fixes: 95c47f9cf5e0 ("ipv4: call dst_dev_put() properly")
> Signed-off-by: Wei Wang <weiwan@google.com>
> Reported-by: Ido Schimmel <idosch@idosch.org>
> Reported-by: Jesse Hathaway <jesse@mbuki-mvuki.org>
> Tested-by: Jesse Hathaway <jesse@mbuki-mvuki.org>
> Acked-by: Martin KaFai Lau <kafai@fb.com>
> Cc: David Ahern <dsahern@gmail.com>

Reviewed-by: Ido Schimmel <idosch@mellanox.com>