Re: [PATCH bpf v5 0/2] Fix bpf_probe_read_user_str() overcopying

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Nov 12, 2020 at 11:13 AM Daniel Xu <dxu@dxuuu.xyz> wrote:
>
> On Wed Nov 11, 2020 at 3:22 PM PST, Andrii Nakryiko wrote:
> > On Wed, Nov 11, 2020 at 2:46 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
> > >
> > > 6ae08ae3dea2 ("bpf: Add probe_read_{user, kernel} and probe_read_{user,
> > > kernel}_str helpers") introduced a subtle bug where
> > > bpf_probe_read_user_str() would potentially copy a few extra bytes after
> > > the NUL terminator.
> > >
> > > This issue is particularly nefarious when strings are used as map keys,
> > > as seemingly identical strings can occupy multiple entries in a map.
> > >
> > > This patchset fixes the issue and introduces a selftest to prevent
> > > future regressions.
> > >
> > > v4 -> v5:
> > > * don't read potentially uninitialized memory
> >
> > I think the bigger problem was that it could overwrite unintended
> > memory. E.g., in BPF program, if you had something like:
> >
> > char my_buf[8 + 3];
> > char my_precious_data[5] = {1, 2, 3, 4, 5};
>
> How does that happen?
>
> The
>
>     while (max >= sizeof(unsigned long)) {
>             /* copy 4 bytes */
>
>             max -= sizeof(unsigned long)
>     }
>
>     /* copy byte at a time */
>
> where `max` is the user supplied length should prevent that kind of
> corruption, right?

Yes, you are right, I got confused. If the user specified the correct
max, then this would have never happened. Never mind.

>
> [...]