Subject: Re: [PATCH bpf-next] bpf: Fix NULL pointer dereference when registering bpf trampoline

From: Jiri Olsa <[email-masked]>
Date: Thu, 28 Jul 2022 15:25:22 +0200

On Thu, Jul 28, 2022 at 08:56:27PM +0800, Xu Kuohai wrote:
> On 7/28/2022 8:27 PM, Jiri Olsa wrote:
> > On Thu, Jul 28, 2022 at 07:40:48AM -0400, Xu Kuohai wrote:
> > > From: Xu Kuohai <[email-masked]>
> > 
> > SNIP
> > 
> > > 
> > > It's caused by a NULL tr->fops passed to ftrace_set_filter_ip(). tr->fops
> > > is initialized to NULL and is assigned to an allocated memory address if
> > > CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS is enabled. Since there is no
> > > direct call on arm64 yet, the config can't be enabled.
> > > 
> > > To fix it, call ftrace_set_filter_ip() only if tr->fops is not NULL.
> > > 
> > > Fixes: 00963a2e75a8 ("bpf: Support bpf_trampoline on functions with IPMODIFY (e.g. livepatch)")
> > > Reported-by: Bruno Goncalves <[email-masked]>
> > > Signed-off-by: Xu Kuohai <[email-masked]>
> > > Tested-by: Bruno Goncalves <[email-masked]>
> > > Acked-by: Song Liu <[email-masked]>
> > > ---
> > >   kernel/bpf/trampoline.c | 11 +++++++++--
> > >   1 file changed, 9 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> > > index 42e387a12694..0d5a9e0b9a7b 100644
> > > --- a/kernel/bpf/trampoline.c
> > > +++ b/kernel/bpf/trampoline.c
> > > @@ -255,8 +255,15 @@ static int register_fentry(struct bpf_trampoline *tr, void *new_addr)
> > >   		return -ENOENT;
> > >   	if (tr->func.ftrace_managed) {
> > > -		ftrace_set_filter_ip(tr->fops, (unsigned long)ip, 0, 0);
> > > -		ret = register_ftrace_direct_multi(tr->fops, (long)new_addr);
> > > +		if (tr->fops)
> > > +			ret = ftrace_set_filter_ip(tr->fops, (unsigned long)ip,
> > > +						   0, 0);
> > > +		else
> > > +			ret = -ENOTSUPP;
> > > +
> > > +		if (!ret)
> > > +			ret = register_ftrace_direct_multi(tr->fops,
> > > +							   (long)new_addr);
> > >   	} else {
> > >   		ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, NULL, new_addr);
> > >   	}
> > 
> > do we need to do the same also in unregister_fentry and modify_fentry ?
> > 
> 
> No need for now, this is the only place where we call ftrace_set_filter_ip().
> 
> tr->fops is passed to ftrace_set_filter_ip() and *ftrace_direct_multi()
> functions, and when CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS is not enabled,
> the *ftrace_direct_multi()s do nothing except returning an error code, so
> it's safe to pass NULL to them.

ok, makes sense

Acked-by: Jiri Olsa <[email-masked]>

jirka