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

From: Jiri Olsa <[email-masked]>
Date: Thu, 25 Mar 2021 22:39:30 +0100

On Thu, Mar 25, 2021 at 10:26:32PM +0100, Daniel Borkmann wrote:
> 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 <[email-masked]>
> > ---
> > 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?

right.. I moved it from the ftrace layer where this was not an option,
so I did not realize bpf_trampoline could get extended, will send new
version

thanks,
jirka

> 
> > +}
> > +
> >   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;
> >   }
> > 
>