Re: [RFC] Use after free in BPF/ XDP during XDP_REDIRECT

["Toke Høiland-Jørgensen" <toke@kernel.org>]

Sebastian Andrzej Siewior <bigeasy@linutronix.de> writes:

> Hi,
>
> Ricardo reported a KASAN related use after free
> 	https://lore.kernel.org/all/20250226-20250204-kasan-slab-use-after-free-read-in-dev_map_enqueue__submit-v3-0-360efec441ba@igalia.com/
>
> in v6.6 stable and suggest a backport of commits
> 	401cb7dae8130 ("net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.")
> 	fecef4cd42c68 ("tun: Assign missing bpf_net_context.")
> 	9da49aa80d686 ("tun: Add missing bpf_net_ctx_clear() in do_xdp_generic()")
>
> as a fix. In the meantime I have the syz reproducer+config and was able
> to investigate.
> It looks as if the syzbot starts a BPF program via xdp_test_run_batch()
> which assigns ri->tgt_value via dev_hash_map_redirect() and the return code
> isn't XDP_REDIRECT it looks like nonsense. So the print in
> bpf_warn_invalid_xdp_action() appears once. Everything goes as planned.
> Then the TUN driver runs another BPF program which returns XDP_REDIRECT
> without setting ri->tgt_value. This appears to be a trick because it
> invoked bpf_trace_printk() which printed four characters. Anyway, this
> is enough to get xdp_do_redirect() going.
>
> The commits in questions do fix it because the bpf_redirect_info becomes
> not only per-task but gets invalidated after the XDP context is left.
>
> Now that I understand it I would suggest something smaller instead as a
> stable fix, (instead the proposed patches). Any objections to the
> following:
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index be313928d272..1d906b7a541d 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -9000,8 +9000,12 @@ static bool xdp_is_valid_access(int off, int size,
>  
>  void bpf_warn_invalid_xdp_action(struct net_device *dev, struct bpf_prog *prog, u32 act)
>  {
> +	struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
>  	const u32 act_max = XDP_REDIRECT;
>  
> +	ri->map_id = INT_MAX;
> +	ri->map_type = BPF_MAP_TYPE_UNSPEC;
> +
>  	pr_warn_once("%s XDP return value %u on prog %s (id %d) dev %s, expect packet loss!\n",
>  		     act > act_max ? "Illegal" : "Driver unsupported",
>  		     act, prog->aux->name, prog->aux->id, dev ? dev->name : "N/A");

From your description above, this will fix the particular error
encountered, but what happens if the initial return code is not in fact
nonsense (so the warn_invalid_action) is not triggered?

I.e.,

bpf_redirect_map(...);
return XDP_DROP;

would still leave ri->map_id and ri->map_type set for the later tun
driver invocation, no?

-Toke