Re: [PATCHv2] bpf: Take module reference for trampoline in module

[Daniel Borkmann <daniel@xxxxxxxxxxxxx>]

On 3/24/21 6:40 PM, Jiri Olsa wrote:
> Currently module can be unloaded even if there's a trampoline
> register in it. It's easily reproduced by running in parallel:
>
>    # while :; do ./test_progs -t module_attach; done
>    # while :; do rmmod bpf_testmod; sleep 0.5; done
>
> Taking the module reference in case the trampoline's ip is
> within the module code. Releasing it when the trampoline's
> ip is unregistered.
>
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
> v2 changes:
>    - fixed ip_module_put to do preempt_disable/preempt_enable
>
>   kernel/bpf/trampoline.c | 31 +++++++++++++++++++++++++++++++
>   1 file changed, 31 insertions(+)
>
> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> index 1f3a4be4b175..39e4280f94e4 100644
> --- a/kernel/bpf/trampoline.c
> +++ b/kernel/bpf/trampoline.c
> @@ -87,6 +87,26 @@ static struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
>   	return tr;
>   }
>   
> +static struct module *ip_module_get(unsigned long ip)
> +{
> +	struct module *mod;
> +	int err = 0;
> +
> +	preempt_disable();
> +	mod = __module_text_address(ip);
> +	if (mod && !try_module_get(mod))
> +		err = -ENOENT;
> +	preempt_enable();
> +	return err ? ERR_PTR(err) : mod;
> +}
> +
> +static void ip_module_put(unsigned long ip)
> +{
> +	preempt_disable();
> +	module_put(__module_text_address(ip));
> +	preempt_enable();

Could we cache the mod pointer in tr instead of doing another addr search
for dropping the ref?

> +}
> +
>   static int is_ftrace_location(void *ip)
>   {
>   	long addr;
> @@ -108,6 +128,9 @@ static int unregister_fentry(struct bpf_trampoline *tr, void *old_addr)
>   		ret = unregister_ftrace_direct((long)ip, (long)old_addr);
>   	else
>   		ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, old_addr, NULL);
> +
> +	if (!ret)
> +		ip_module_put((unsigned long) ip);
>   	return ret;
>   }
>   
> @@ -126,6 +149,7 @@ static int modify_fentry(struct bpf_trampoline *tr, void *old_addr, void *new_ad
>   /* first time registering */
>   static int register_fentry(struct bpf_trampoline *tr, void *new_addr)
>   {
> +	struct module *mod;
>   	void *ip = tr->func.addr;
>   	int ret;
>   
> @@ -134,10 +158,17 @@ static int register_fentry(struct bpf_trampoline *tr, void *new_addr)
>   		return ret;
>   	tr->func.ftrace_managed = ret;
>   
> +	mod = ip_module_get((unsigned long) ip);
> +	if (IS_ERR(mod))
> +		return -ENOENT;
> +
>   	if (tr->func.ftrace_managed)
>   		ret = register_ftrace_direct((long)ip, (long)new_addr);
>   	else
>   		ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, NULL, new_addr);
> +
> +	if (ret)
> +		module_put(mod);
>   	return ret;
>   }
>