[PATCH net] ipv6: Fix null-ptr-deref in fib6_nh_mtu_change().

[PATCH net] ipv6: Fix null-ptr-deref in fib6_nh_mtu_change().

[xmei5]

From: Xiang Mei <xmei5@asu.edu>

fib6_nh_mtu_change() re-fetches idev via __in6_dev_get(arg->dev) and
dereferences idev->cnf.mtu6 without a NULL check. addrconf_ifdown()
clears dev->ip6_ptr with RCU_INIT_POINTER() after rt6_disable_ip() has
released tb6_lock, so the RA-driven MTU walk can observe a NULL idev and
oops. The caller rt6_mtu_change_route() guards its own __in6_dev_get(),
but this re-fetch is unguarded; nexthop-backed routes survive
addrconf_ifdown()'s flush, so the walk still reaches it after ip6_ptr is
nulled.

Return 0 when idev is NULL, matching rt6_mtu_change_route() and the
fib6_mtu() fix in commit 5ad509c1fdad ("ipv6: Fix null-ptr-deref in
fib6_mtu().").

  Oops: general protection fault, ... KASAN: null-ptr-deref in range
        [0x00000000000002a8-0x00000000000002af]
  RIP: 0010:fib6_nh_mtu_change+0x203/0x990
   rt6_mtu_change_route+0x141/0x1d0
   __fib6_clean_all+0xd0/0x160
   rt6_mtu_change+0xb4/0x100
   ndisc_router_discovery+0x24b5/0x2cb0
   icmpv6_rcv+0x12e9/0x1710
   ipv6_rcv+0x39b/0x410

Fixes: c0b220cf7d80 ("ipv6: Refactor exception functions")
Reported-by: Weiming Shi <bestswngs@gmail.com>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Xiang Mei <xmei5@asu.edu>
---
 net/ipv6/route.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 6361ad2fcf77..a1301334da48 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -5055,6 +5055,9 @@ static int fib6_nh_mtu_change(struct fib6_nh *nh, void *_arg)
 		struct inet6_dev *idev = __in6_dev_get(arg->dev);
 		u32 mtu = f6i->fib6_pmtu;
 
+		if (!idev)
+			return 0;
+
 		if (mtu >= arg->mtu ||
 		    (mtu < arg->mtu && mtu == idev->cnf.mtu6))
 			fib6_metric_set(f6i, RTAX_MTU, arg->mtu);
-- 
2.43.0

Re: [PATCH net] ipv6: Fix null-ptr-deref in fib6_nh_mtu_change().

[Fernando Fernandez Mancera]

On 6/19/26 6:53 AM, xmei5@asu.edu wrote:
> From: Xiang Mei <xmei5@asu.edu>
> 
> fib6_nh_mtu_change() re-fetches idev via __in6_dev_get(arg->dev) and
> dereferences idev->cnf.mtu6 without a NULL check. addrconf_ifdown()
> clears dev->ip6_ptr with RCU_INIT_POINTER() after rt6_disable_ip() has
> released tb6_lock, so the RA-driven MTU walk can observe a NULL idev and
> oops. The caller rt6_mtu_change_route() guards its own __in6_dev_get(),
> but this re-fetch is unguarded; nexthop-backed routes survive
> addrconf_ifdown()'s flush, so the walk still reaches it after ip6_ptr is
> nulled.
> 
> Return 0 when idev is NULL, matching rt6_mtu_change_route() and the
> fib6_mtu() fix in commit 5ad509c1fdad ("ipv6: Fix null-ptr-deref in
> fib6_mtu().").
> 
>    Oops: general protection fault, ... KASAN: null-ptr-deref in range
>          [0x00000000000002a8-0x00000000000002af]
>    RIP: 0010:fib6_nh_mtu_change+0x203/0x990
>     rt6_mtu_change_route+0x141/0x1d0
>     __fib6_clean_all+0xd0/0x160
>     rt6_mtu_change+0xb4/0x100
>     ndisc_router_discovery+0x24b5/0x2cb0
>     icmpv6_rcv+0x12e9/0x1710
>     ipv6_rcv+0x39b/0x410
> 
> Fixes: c0b220cf7d80 ("ipv6: Refactor exception functions")
> Reported-by: Weiming Shi <bestswngs@gmail.com>
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Xiang Mei <xmei5@asu.edu>

Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>

Thanks!

Re: [PATCH net] ipv6: Fix null-ptr-deref in fib6_nh_mtu_change().

[Ido Schimmel]

On Thu, Jun 18, 2026 at 09:53:34PM -0700, xmei5@asu.edu wrote:
> From: Xiang Mei <xmei5@asu.edu>
> 
> fib6_nh_mtu_change() re-fetches idev via __in6_dev_get(arg->dev) and
> dereferences idev->cnf.mtu6 without a NULL check. addrconf_ifdown()
> clears dev->ip6_ptr with RCU_INIT_POINTER() after rt6_disable_ip() has
> released tb6_lock, so the RA-driven MTU walk can observe a NULL idev and
> oops. The caller rt6_mtu_change_route() guards its own __in6_dev_get(),
> but this re-fetch is unguarded; nexthop-backed routes survive
> addrconf_ifdown()'s flush, so the walk still reaches it after ip6_ptr is
> nulled.
> 
> Return 0 when idev is NULL, matching rt6_mtu_change_route() and the
> fib6_mtu() fix in commit 5ad509c1fdad ("ipv6: Fix null-ptr-deref in
> fib6_mtu().").
> 
>   Oops: general protection fault, ... KASAN: null-ptr-deref in range
>         [0x00000000000002a8-0x00000000000002af]
>   RIP: 0010:fib6_nh_mtu_change+0x203/0x990
>    rt6_mtu_change_route+0x141/0x1d0
>    __fib6_clean_all+0xd0/0x160
>    rt6_mtu_change+0xb4/0x100
>    ndisc_router_discovery+0x24b5/0x2cb0
>    icmpv6_rcv+0x12e9/0x1710
>    ipv6_rcv+0x39b/0x410
> 
> Fixes: c0b220cf7d80 ("ipv6: Refactor exception functions")
> Reported-by: Weiming Shi <bestswngs@gmail.com>
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Xiang Mei <xmei5@asu.edu>

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