linux-kernel - Re: [PATCH bpf-next] bpf: Allow bpf_d_path in perf_event

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Date:   Mon, 1 Nov 2021 20:20:17 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Yonghong Song <yhs@...com>, Florent Revest <revest@...omium.org>,
        Hengqi Chen <hengqi.chen@...il.com>,
        Martin KaFai Lau <kafai@...com>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        KP Singh <kpsingh@...nel.org>,
        Brendan Jackman <jackmanb@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next] bpf: Allow bpf_d_path in perf_event_mmap

On Mon, Nov 1, 2021 at 8:16 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
> > >
> > >     FILE *vm_file = vma->vm_file; /* no checking is needed, vma from
> > > parameter which is not NULL */
> > >     if (vm_file)
> > >       bpf_d_path(&vm_file->f_path, path, sizeof(path));
> >
> > That should work.
> > The verifier can achieve that by marking certain fields as PTR_TO_BTF_ID_OR_NULL
> > instead of PTR_TO_BTF_ID while walking such pointers.
> > And then disallow pointer arithmetic on PTR_TO_BTF_ID_OR_NULL until it
> > goes through 'if (Rx == NULL)' check inside the program and gets converted to
> > PTR_TO_BTF_ID.
> > Initially we can hard code such fields via BTF_ID(struct, file) macro.'
> > So any pointer that results into a 'struct file' pointer will be
> > PTR_TO_BTF_ID_OR_NULL.
>
> Can we just require all helpers to check NULL if they accept
> PTR_TO_BTF_ID? It's always been a case that PTR_TO_BTF_ID can be null.
> We should audit all the helpers with ARG_PTR_TO_BTF_ID and ensure they
> do proper validation, of course.
>
> Or am I missing the essence of the issue?

It's not a pointer dereference. It's math on the pointer. The
&vm_file->f_path part.
The helper can check that it's [0, few_pages] and declare it's bad.
I guess we can do that and only do what I proposed for "more than a page"
math on the pointer. Or even disallow "add more than a page offset to
PTR_TO_BTF_ID"
for now, since it will cover 99% of the cases.