netdev - Re: [PATCHv2] bpf: Add deny list of btf ids check for tracing programs

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Message-ID: <CAADnVQLJzJinUhWM4eFd1=GEjgNT-25y_bCx7LOjhpSXumKcHw@mail.gmail.com>
Date:   Tue, 11 May 2021 14:05:54 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [PATCHv2] bpf: Add deny list of btf ids check for tracing programs

On Mon, May 10, 2021 at 2:50 AM Jiri Olsa <jolsa@...hat.com> wrote:
>
> On Thu, May 06, 2021 at 06:36:38PM -0700, Alexei Starovoitov wrote:
> > On Thu, Apr 29, 2021 at 4:47 AM Jiri Olsa <jolsa@...nel.org> wrote:
> > >
> > > The recursion check in __bpf_prog_enter and __bpf_prog_exit
> > > leaves some (not inlined) functions unprotected:
> > >
> > > In __bpf_prog_enter:
> > >   - migrate_disable is called before prog->active is checked
> > >
> > > In __bpf_prog_exit:
> > >   - migrate_enable,rcu_read_unlock_strict are called after
> > >     prog->active is decreased
> > >
> > > When attaching trampoline to them we get panic like:
> > >
> > >   traps: PANIC: double fault, error_code: 0x0
> > >   double fault: 0000 [#1] SMP PTI
> > >   RIP: 0010:__bpf_prog_enter+0x4/0x50
> > >   ...
> > >   Call Trace:
> > >    <IRQ>
> > >    bpf_trampoline_6442466513_0+0x18/0x1000
> > >    migrate_disable+0x5/0x50
> > >    __bpf_prog_enter+0x9/0x50
> > >    bpf_trampoline_6442466513_0+0x18/0x1000
> > >    migrate_disable+0x5/0x50
> > >    __bpf_prog_enter+0x9/0x50
> > >    bpf_trampoline_6442466513_0+0x18/0x1000
> > >    migrate_disable+0x5/0x50
> > >    __bpf_prog_enter+0x9/0x50
> > >    bpf_trampoline_6442466513_0+0x18/0x1000
> > >    migrate_disable+0x5/0x50
> > >    ...
> > >
> > > Fixing this by adding deny list of btf ids for tracing
> > > programs and checking btf id during program verification.
> > > Adding above functions to this list.
> > >
> > > Suggested-by: Alexei Starovoitov <ast@...nel.org>
> > > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> > > ---
> > > v2 changes:
> > >   - drop check for EXT programs [Andrii]
> > >
> > >  kernel/bpf/verifier.c | 14 ++++++++++++++
> > >  1 file changed, 14 insertions(+)
> > >
> > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > index 2579f6fbb5c3..42311e51ac71 100644
> > > --- a/kernel/bpf/verifier.c
> > > +++ b/kernel/bpf/verifier.c
> > > @@ -13112,6 +13112,17 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
> > >         return 0;
> > >  }
> > >
> > > +BTF_SET_START(btf_id_deny)
> > > +BTF_ID_UNUSED
> > > +#ifdef CONFIG_SMP
> > > +BTF_ID(func, migrate_disable)
> > > +BTF_ID(func, migrate_enable)
> > > +#endif
> > > +#if !defined CONFIG_PREEMPT_RCU && !defined CONFIG_TINY_RCU
> > > +BTF_ID(func, rcu_read_unlock_strict)
> > > +#endif
> > > +BTF_SET_END(btf_id_deny)
> >
> > I was wondering whether it makes sense to do this on pahole side instead ?
> > It can do more flexible regex matching and excluding all such functions
> > from vmlinux btf without the kernel having to do a maze of #ifdef
> > depending on config.
> > On one side we will lose BTF info about such functions, but what do we
> > need it for?
> > On the other side it will be a tiny reduction in vmlinux btf :)
> > Thoughts?
>
> we just removed the ftrace filter so BTF will have 'all' functions
>
> I think the filtering on pahole side could cause problems like
> the recent one with cubictcp_state.. it's just 3 functions, but
> what if they rename? this way we at least get compilation error ;-)
>
> I'd go with all functions in BTF and restrict attachment for those
> that cause problems

Ok. Let's see how it will work in practice.
Applied to bpf tree.