Re: [PATCH net] ipv4: stop PMTU walk when nexthop group shrinks

[Ido Schimmel]

On Thu, Oct 01, 2026 at 10:05:50AM +0900, Daehyeon Ko wrote:
> 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@xxxxxxxxxxxxxxx
> Assisted-by: LLM
> Signed-off-by: Daehyeon Ko <4ncienth@xxxxxxxxx>

Reviewed-by: Ido Schimmel <idosch@xxxxxxxxxx>

Please check fib_dump_info_fnhe(). It seems to suffer from the same
problem since commit 4ce5dc9316de ("inet: switch inet_dump_fib() to RCU
protection").