Re: [PATCH] netfilter: nf_conntrack_helper: pass helper to expect cleanup

[Phil Sutter <phil@nwl.cc>]

On Mon, Mar 30, 2026 at 12:50:36AM +0800, Qi Tang wrote:
> nf_conntrack_helper_unregister() calls nf_ct_expect_iterate_destroy()
> to remove expectations belonging to the helper being unregistered.
> However, it passes NULL instead of the helper pointer as the data
> argument, so expect_iter_me() never matches any expectation and all
> of them survive the cleanup.
> 
> After unregister returns, nfnl_cthelper_del() frees the helper
> object immediately.  Subsequent expectation dumps or packet-driven
> init_conntrack() calls then dereference the freed exp->helper,
> causing a use-after-free.
> 
> Pass the actual helper pointer so expectations referencing it are
> properly destroyed before the helper object is freed.
> 
>   BUG: KASAN: slab-use-after-free in string+0x38f/0x430
>   Read of size 1 at addr ffff888003b14d20 by task poc/103
>   Call Trace:
>    string+0x38f/0x430
>    vsnprintf+0x3cc/0x1170
>    seq_printf+0x17a/0x240
>    exp_seq_show+0x2e5/0x560
>    seq_read_iter+0x419/0x1280
>    proc_reg_read+0x1ac/0x270
>    vfs_read+0x179/0x930
>    ksys_read+0xef/0x1c0
>   Freed by task 103:
>   The buggy address is located 32 bytes inside of
>    freed 192-byte region [ffff888003b14d00, ffff888003b14dc0)
> 
> Fixes: ac7b84839003 ("netfilter: expect: add and use nf_ct_expect_iterate helpers")
> Signed-off-by: Qi Tang <tpluszz77@gmail.com>

Reviewed-by: Phil Sutter <phil@nwl.cc>

Thanks, Phil