netdev - Re: [PATCH V2] net: rose: Prevent the use of freed digipeat

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Message-ID: <CANn89iKS3ZOSva0EUjLFD+CmJeT=JgX3-7bHxgHChMMQpx+7=g@mail.gmail.com>
Date: Fri, 24 Oct 2025 02:19:45 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Lizhi Xu <lizhi.xu@...driver.com>
Cc: kuniyu@...gle.com, davem@...emloft.net, horms@...nel.org, jreuter@...na.de, 
	kuba@...nel.org, linux-hams@...r.kernel.org, linux-kernel@...r.kernel.org, 
	netdev@...r.kernel.org, pabeni@...hat.com, 
	syzbot+caa052a0958a9146870d@...kaller.appspotmail.com, 
	syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH V2] net: rose: Prevent the use of freed digipeat

On Fri, Oct 24, 2025 at 2:06 AM Lizhi Xu <lizhi.xu@...driver.com> wrote:
>
> There is no synchronization between the two timers, rose_t0timer_expiry
> and rose_timer_expiry.
> rose_timer_expiry() puts the neighbor when the rose state is ROSE_STATE_2.
> However, rose_t0timer_expiry() does initiate a restart request on the
> neighbor.
> When rose_t0timer_expiry() accesses the released neighbor member digipeat,
> a UAF is triggered.
>
> To avoid this UAF, defer the put operation to rose_t0timer_expiry() and
> stop restarting t0timer after putting the neighbor.
>
> When putting the neighbor, set the neighbor to NULL. Setting neighbor to
> NULL prevents rose_t0timer_expiry() from restarting t0timer.
>
> syzbot reported a slab-use-after-free Read in ax25_find_cb.
> BUG: KASAN: slab-use-after-free in ax25_find_cb+0x3b8/0x3f0 net/ax25/af_ax25.c:237
> Read of size 1 at addr ffff888059c704c0 by task syz.6.2733/17200
> Call Trace:
>  ax25_find_cb+0x3b8/0x3f0 net/ax25/af_ax25.c:237
>  ax25_send_frame+0x157/0xb60 net/ax25/ax25_out.c:55
>  rose_send_frame+0xcc/0x2c0 net/rose/rose_link.c:106
>  rose_transmit_restart_request+0x1b8/0x240 net/rose/rose_link.c:198
>  rose_t0timer_expiry+0x1d/0x150 net/rose/rose_link.c:83
>
> Freed by task 17183:
>  kfree+0x2b8/0x6d0 mm/slub.c:6826
>  rose_neigh_put include/net/rose.h:165 [inline]
>  rose_timer_expiry+0x537/0x630 net/rose/rose_timer.c:183
>
> Fixes: d860d1faa6b2 ("net: rose: convert 'use' field to refcount_t")
> Reported-by: syzbot+caa052a0958a9146870d@...kaller.appspotmail.com
> Signed-off-by: Lizhi Xu <lizhi.xu@...driver.com>
> ---
> V1 -> V2: Putting the neighbor stops t0timer from automatically starting
>
>  include/net/rose.h   | 1 +
>  net/rose/rose_link.c | 5 +++++
>  2 files changed, 6 insertions(+)
>
> diff --git a/include/net/rose.h b/include/net/rose.h
> index 2b5491bbf39a..ecf37c8e24bb 100644
> --- a/include/net/rose.h
> +++ b/include/net/rose.h
> @@ -164,6 +164,7 @@ static inline void rose_neigh_put(struct rose_neigh *rose_neigh)
>                         ax25_cb_put(rose_neigh->ax25);
>                 kfree(rose_neigh->digipeat);
>                 kfree(rose_neigh);
> +               rose_neigh = NULL;

What is the purpose of this added line ?

@rose_neigh is a local variable. Setting it to NULL while this
function no longer uses it is
going to be optimized out by the compiler. Even if not optimized, this
has no effect.


>         }
>  }
>
> diff --git a/net/rose/rose_link.c b/net/rose/rose_link.c
> index 7746229fdc8c..524e7935bd02 100644
> --- a/net/rose/rose_link.c
> +++ b/net/rose/rose_link.c
> @@ -43,6 +43,9 @@ void rose_start_ftimer(struct rose_neigh *neigh)
>
>  static void rose_start_t0timer(struct rose_neigh *neigh)
>  {
> +       if (!neigh)
> +               return;

This will never fire. callers would have crashed already it neigh was NULL.

> +
>         timer_delete(&neigh->t0timer);
>
>         neigh->t0timer.function = rose_t0timer_expiry;
> @@ -80,10 +83,12 @@ static void rose_t0timer_expiry(struct timer_list *t)
>  {
>         struct rose_neigh *neigh = timer_container_of(neigh, t, t0timer);
>

Can you explain (in a comment) why this is needed ?
What is the precise scenario you want to avoid ?

> +       rose_neigh_hold(neigh);
>         rose_transmit_restart_request(neigh);
>
>         neigh->dce_mode = 0;
>
> +       rose_neigh_put(neigh);

I am pretty sure this patch fixes nothing at all.

>         rose_start_t0timer(neigh);
>  }
>
> --
> 2.43.0
>