[PATCH net v1] net: nexthop: fix panic when IPv4 route references IPv6 nexthop

[PATCH net v1] net: nexthop: fix panic when IPv4 route references IPv6 nexthop

[Jiayuan Chen]

From: Jiayuan Chen <jiayuan.chen@shopee.com>

fib_check_nexthop() does not validate that the nexthop family matches
the route family. This allows an IPv4 route to reference an IPv6
nexthop object. When the IPv4 route is looked up, __mkroute_output()
accesses nhc->nhc_pcpu_rth_output which is never allocated for IPv6
nexthops (fib6_nh_init does not call fib_nh_common_init), causing a
NULL pointer dereference.

Note that this is not about IPv4 routes with IPv6 gateways (RFC 5549),
which uses an AF_INET nexthop with nhc_gw_family=AF_INET6 and properly
allocates nhc_pcpu_rth_output via fib_nh_common_init(). The bug here
is an AF_INET6 nexthop object being directly referenced by an IPv4
route, which is an invalid combination.

Add the missing family check in fib_check_nexthop(), mirroring what
fib6_check_nexthop() already does for the reverse direction (rejecting
IPv6 routes that reference IPv4 nexthop objects).

Reproducer:

  unshare -rn
  ip link set lo up
  ip nexthop add id 100 via fe80::1 dev lo
  ip route add 172.20.20.0/24 nhid 100
  ping -c1 172.20.20.1

After fix:
...
$ ip route add 172.20.20.0/24 nhid 100
Error: IPv4 routes can not use an IPv6 nexthop.

Reported-by: syzbot+334190e097a98a1b81bb@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/698f8482.a70a0220.2c38d7.00ca.GAE@google.com/T/
Fixes: 4c7e8084fd46 ("ipv4: Plumb support for nexthop object in a fib_info")
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
---
 net/ipv4/nexthop.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c
index 7b9d70f9b31c..0f236110cd58 100644
--- a/net/ipv4/nexthop.c
+++ b/net/ipv4/nexthop.c
@@ -1634,6 +1634,12 @@ int fib_check_nexthop(struct nexthop *nh, u8 scope,
 			goto out;
 		}
 
+		if (!nhg->has_v4) {
+			NL_SET_ERR_MSG(extack, "IPv4 routes can not use an IPv6 nexthop");
+			err = -EINVAL;
+			goto out;
+		}
+
 		if (scope == RT_SCOPE_HOST) {
 			NL_SET_ERR_MSG(extack, "Route with host scope can not have multiple nexthops");
 			err = -EINVAL;
@@ -1650,6 +1656,11 @@ int fib_check_nexthop(struct nexthop *nh, u8 scope,
 			err = -EINVAL;
 			goto out;
 		}
+		if (nhi->family != AF_INET) {
+			NL_SET_ERR_MSG(extack, "IPv4 routes can not use an IPv6 nexthop");
+			err = -EINVAL;
+			goto out;
+		}
 		err = nexthop_check_scope(nhi, scope, extack);
 	}
 
-- 
2.43.0

Re: [PATCH net v1] net: nexthop: fix panic when IPv4 route references IPv6 nexthop

[Jakub Kicinski]

On Sat, 28 Feb 2026 11:13:59 +0800 Jiayuan Chen wrote:
> From: Jiayuan Chen <jiayuan.chen@shopee.com>
> 
> fib_check_nexthop() does not validate that the nexthop family matches
> the route family. This allows an IPv4 route to reference an IPv6
> nexthop object. When the IPv4 route is looked up, __mkroute_output()
> accesses nhc->nhc_pcpu_rth_output which is never allocated for IPv6
> nexthops (fib6_nh_init does not call fib_nh_common_init), causing a
> NULL pointer dereference.
> 
> Note that this is not about IPv4 routes with IPv6 gateways (RFC 5549),
> which uses an AF_INET nexthop with nhc_gw_family=AF_INET6 and properly
> allocates nhc_pcpu_rth_output via fib_nh_common_init(). The bug here
> is an AF_INET6 nexthop object being directly referenced by an IPv4
> route, which is an invalid combination.
> 
> Add the missing family check in fib_check_nexthop(), mirroring what
> fib6_check_nexthop() already does for the reverse direction (rejecting
> IPv6 routes that reference IPv4 nexthop objects).

AFAICT this breaks a bunch of tests, quickest to repro with is
gre_multipath_nh.sh but you should probably run fib_nexthops.sh
on your fix as well.

> Reproducer:
> 
>   unshare -rn
>   ip link set lo up
>   ip nexthop add id 100 via fe80::1 dev lo
>   ip route add 172.20.20.0/24 nhid 100
>   ping -c1 172.20.20.1
-- 
pw-bot: cr

Re: [PATCH net v1] net: nexthop: fix panic when IPv4 route references IPv6 nexthop

[David Ahern]

On 2/28/26 8:39 AM, Jakub Kicinski wrote:
> On Sat, 28 Feb 2026 11:13:59 +0800 Jiayuan Chen wrote:
>> From: Jiayuan Chen <jiayuan.chen@shopee.com>
>>
>> fib_check_nexthop() does not validate that the nexthop family matches
>> the route family. This allows an IPv4 route to reference an IPv6
>> nexthop object. When the IPv4 route is looked up, __mkroute_output()
>> accesses nhc->nhc_pcpu_rth_output which is never allocated for IPv6
>> nexthops (fib6_nh_init does not call fib_nh_common_init), causing a
>> NULL pointer dereference.
>>
>> Note that this is not about IPv4 routes with IPv6 gateways (RFC 5549),
>> which uses an AF_INET nexthop with nhc_gw_family=AF_INET6 and properly
>> allocates nhc_pcpu_rth_output via fib_nh_common_init(). The bug here
>> is an AF_INET6 nexthop object being directly referenced by an IPv4
>> route, which is an invalid combination.
>>
>> Add the missing family check in fib_check_nexthop(), mirroring what
>> fib6_check_nexthop() already does for the reverse direction (rejecting
>> IPv6 routes that reference IPv4 nexthop objects).
> 
> AFAICT this breaks a bunch of tests, quickest to repro with is
> gre_multipath_nh.sh but you should probably run fib_nexthops.sh
> on your fix as well.
> 

nothing to fix. The patch is wrong. IPv4 supports IPv6 gateways; that is
a known feature.

please post the stack trace for the panic

Re: [PATCH net v1] net: nexthop: fix panic when IPv4 route references IPv6 nexthop

[Eric Dumazet]

On Sat, Feb 28, 2026 at 5:33 PM David Ahern <dsahern@kernel.org> wrote:
>
> On 2/28/26 8:39 AM, Jakub Kicinski wrote:
> > On Sat, 28 Feb 2026 11:13:59 +0800 Jiayuan Chen wrote:
> >> From: Jiayuan Chen <jiayuan.chen@shopee.com>
> >>
> >> fib_check_nexthop() does not validate that the nexthop family matches
> >> the route family. This allows an IPv4 route to reference an IPv6
> >> nexthop object. When the IPv4 route is looked up, __mkroute_output()
> >> accesses nhc->nhc_pcpu_rth_output which is never allocated for IPv6
> >> nexthops (fib6_nh_init does not call fib_nh_common_init), causing a
> >> NULL pointer dereference.
> >>
> >> Note that this is not about IPv4 routes with IPv6 gateways (RFC 5549),
> >> which uses an AF_INET nexthop with nhc_gw_family=AF_INET6 and properly
> >> allocates nhc_pcpu_rth_output via fib_nh_common_init(). The bug here
> >> is an AF_INET6 nexthop object being directly referenced by an IPv4
> >> route, which is an invalid combination.
> >>
> >> Add the missing family check in fib_check_nexthop(), mirroring what
> >> fib6_check_nexthop() already does for the reverse direction (rejecting
> >> IPv6 routes that reference IPv4 nexthop objects).
> >
> > AFAICT this breaks a bunch of tests, quickest to repro with is
> > gre_multipath_nh.sh but you should probably run fib_nexthops.sh
> > on your fix as well.
> >
>
> nothing to fix. The patch is wrong. IPv4 supports IPv6 gateways; that is
> a known feature.
>
> please post the stack trace for the panic

https://lore.kernel.org/all/698f8482.a70a0220.2c38d7.00ca.GAE@google.com/T/

Re: [PATCH net v1] net: nexthop: fix panic when IPv4 route references IPv6 nexthop

[Jiayuan Chen]

March 1, 2026 at 01:04, "Eric Dumazet" <edumazet@google.com mailto:edumazet@google.com?to=%22Eric%20Dumazet%22%20%3Cedumazet%40google.com%3E > wrote:


> 
> On Sat, Feb 28, 2026 at 5:33 PM David Ahern <dsahern@kernel.org> wrote:
> 
> > 
> > On 2/28/26 8:39 AM, Jakub Kicinski wrote:
> >  On Sat, 28 Feb 2026 11:13:59 +0800 Jiayuan Chen wrote:
> >  From: Jiayuan Chen <jiayuan.chen@shopee.com>
> > 
> >  fib_check_nexthop() does not validate that the nexthop family matches
> >  the route family. This allows an IPv4 route to reference an IPv6
> >  nexthop object. When the IPv4 route is looked up, __mkroute_output()
> >  accesses nhc->nhc_pcpu_rth_output which is never allocated for IPv6
> >  nexthops (fib6_nh_init does not call fib_nh_common_init), causing a
> >  NULL pointer dereference.
> > 
> >  Note that this is not about IPv4 routes with IPv6 gateways (RFC 5549),
> >  which uses an AF_INET nexthop with nhc_gw_family=AF_INET6 and properly
> >  allocates nhc_pcpu_rth_output via fib_nh_common_init(). The bug here
> >  is an AF_INET6 nexthop object being directly referenced by an IPv4
> >  route, which is an invalid combination.
> > 
> >  Add the missing family check in fib_check_nexthop(), mirroring what
> >  fib6_check_nexthop() already does for the reverse direction (rejecting
> >  IPv6 routes that reference IPv4 nexthop objects).
> > 
> >  AFAICT this breaks a bunch of tests, quickest to repro with is
> >  gre_multipath_nh.sh but you should probably run fib_nexthops.sh
> >  on your fix as well.
> > 
> >  nothing to fix. The patch is wrong. IPv4 supports IPv6 gateways; that is
> >  a known feature.
> > 
> >  please post the stack trace for the panic
> > 
> https://lore.kernel.org/all/698f8482.a70a0220.2c38d7.00ca.GAE@google.com/T/
>


My bad, the previous fix was wrong - IPv4 routes referencing IPv6
nexthop objects is totally via this path.

The crash actually only happens with loopback nexthops, e.g.:

    ip nexthop add id 100 via fe80::1 dev lo

In fib6_nh_init(), nexthop objects always have fc_dst=:: (no
destination prefix), so fib6_is_reject() returns true for any
nexthop using loopback device. This causes it to skip
fib_nh_common_init(), leaving nhc_pcpu_rth_output, nhc_exceptions
and nhc_rth_input all NULL. When an IPv4 route later references
this nexthop, __mkroute_output() hits raw_cpu_ptr(NULL) and crashes.

The simplest fix is just allocating nhc_pcpu_rth_output in the
reject path of fib6_nh_init(). The release path already handles
it correctly.


diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index c0350d97307e..4e7c44101709 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -3643,6 +3643,12 @@ int fib6_nh_init(struct net *net, struct fib6_nh *fib6_nh,
                                goto out;
                        }
                }
+               fib6_nh->nh_common.nhc_pcpu_rth_output =
+                       alloc_percpu_gfp(struct rtable __rcu *, gfp_flags);
+               if (!fib6_nh->nh_common.nhc_pcpu_rth_output) {
+                       err = -ENOMEM;
+                       goto out;
+               }
                goto pcpu_alloc;
        }


./fib_nexthops.sh
Tests passed: 244
Tests failed:   0
Tests skipped:  2
root@bms-ytl-d1-ap

Re: [PATCH net v1] net: nexthop: fix panic when IPv4 route references IPv6 nexthop

[David Ahern]

On 2/28/26 6:57 PM, Jiayuan Chen wrote:
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index c0350d97307e..4e7c44101709 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -3643,6 +3643,12 @@ int fib6_nh_init(struct net *net, struct fib6_nh *fib6_nh,
>                                 goto out;
>                         }
>                 }
> +               fib6_nh->nh_common.nhc_pcpu_rth_output =
> +                       alloc_percpu_gfp(struct rtable __rcu *, gfp_flags);
> +               if (!fib6_nh->nh_common.nhc_pcpu_rth_output) {
> +                       err = -ENOMEM;
> +                       goto out;
> +               }
>                 goto pcpu_alloc;
>         }
> 
>

Already done by  fib6_nh_init -> fib_nh_common_init():

        nhc->nhc_pcpu_rth_output = alloc_percpu_gfp(struct rtable __rcu *,
                                                    gfp_flags);
        if (!nhc->nhc_pcpu_rth_output)
                return -ENOMEM;

Re: [PATCH net v1] net: nexthop: fix panic when IPv4 route references IPv6 nexthop

[David Ahern]

On 2/28/26 6:57 PM, Jiayuan Chen wrote:
> The crash actually only happens with loopback nexthops, e.g.:
> 
>     ip nexthop add id 100 via fe80::1 dev lo
> 
> In fib6_nh_init(), nexthop objects always have fc_dst=:: (no
> destination prefix), so fib6_is_reject() returns true for any
> nexthop using loopback device. This causes it to skip
> fib_nh_common_init(), leaving nhc_pcpu_rth_output, nhc_exceptions
> and nhc_rth_input all NULL. When an IPv4 route later references
> this nexthop, __mkroute_output() hits raw_cpu_ptr(NULL) and crashes.
> 
> The simplest fix is just allocating nhc_pcpu_rth_output in the
> reject path of fib6_nh_init(). The release path already handles
> it correctly.

should have read this before that last response. fib_nh_common_init
exists to avoid putting ipv4 logic in ipv6 code, so your proposed patch
is wrong.

Perhaps the better change is to not use fib6_is_reject in fib6_nh_init
or relax the loopback check or move the jump label pcpu_alloc to still
call fib_nh_common_init.

Also, a test should be added to the nexthops test for this case.