netdev - Re: [PATCH net] nexthop: Fix division by zero while replacing a resilient group

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Date:   Sun, 19 Sep 2021 11:08:11 -0600
From:   David Ahern <dsahern@...il.com>
To:     Ido Schimmel <idosch@...sch.org>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, kuba@...nel.org, petrm@...dia.com,
        mlxsw@...dia.com, Ido Schimmel <idosch@...dia.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH net] nexthop: Fix division by zero while replacing a
 resilient group

On 9/17/21 7:02 AM, Ido Schimmel wrote:
> From: Ido Schimmel <idosch@...dia.com>
> 
> The resilient nexthop group torture tests in fib_nexthop.sh exposed a
> possible division by zero while replacing a resilient group [1]. The
> division by zero occurs when the data path sees a resilient nexthop
> group with zero buckets.
> 
> The tests replace a resilient nexthop group in a loop while traffic is
> forwarded through it. The tests do not specify the number of buckets
> while performing the replacement, resulting in the kernel allocating a
> stub resilient table (i.e, 'struct nh_res_table') with zero buckets.
> 
> This table should never be visible to the data path, but the old nexthop
> group (i.e., 'oldg') might still be used by the data path when the stub
> table is assigned to it.
> 
> Fix this by only assigning the stub table to the old nexthop group after
> making sure the group is no longer used by the data path.
> 
> Tested with fib_nexthops.sh:
> 
> Tests passed: 222
> Tests failed:   0
> 
> [1]
>  divide error: 0000 [#1] PREEMPT SMP KASAN
>  CPU: 0 PID: 1850 Comm: ping Not tainted 5.14.0-custom-10271-ga86eb53057fe #1107
>  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-4.fc34 04/01/2014
>  RIP: 0010:nexthop_select_path+0x2d2/0x1a80
> [...]
>  Call Trace:
>   fib_select_multipath+0x79b/0x1530
>   fib_select_path+0x8fb/0x1c10
>   ip_route_output_key_hash_rcu+0x1198/0x2da0
>   ip_route_output_key_hash+0x190/0x340
>   ip_route_output_flow+0x21/0x120
>   raw_sendmsg+0x91d/0x2e10
>   inet_sendmsg+0x9e/0xe0
>   __sys_sendto+0x23d/0x360
>   __x64_sys_sendto+0xe1/0x1b0
>   do_syscall_64+0x35/0x80
>   entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> Cc: stable@...r.kernel.org
> Fixes: 283a72a5599e ("nexthop: Add implementation of resilient next-hop groups")
> Signed-off-by: Ido Schimmel <idosch@...dia.com>
> Reviewed-by: Petr Machata <petrm@...dia.com>
> ---
>  net/ipv4/nexthop.c | 2 ++
>  1 file changed, 2 insertions(+)
> 

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