Re: [PATCH v5 bpf-next 01/10] lib/buildid: harden build ID parsing logic

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

On Tue, Aug 13, 2024 at 1:59 PM Jann Horn <jannh@google.com> wrote:
>
> On Tue, Aug 13, 2024 at 2:29 AM Andrii Nakryiko <andrii@kernel.org> wrote:
> > Harden build ID parsing logic, adding explicit READ_ONCE() where it's
> > important to have a consistent value read and validated just once.
> >
> > Also, as pointed out by Andi Kleen, we need to make sure that entire ELF
> > note is within a page bounds, so move the overflow check up and add an
> > extra note_size boundaries validation.
> >
> > Fixes tag below points to the code that moved this code into
> > lib/buildid.c, and then subsequently was used in perf subsystem, making
> > this code exposed to perf_event_open() users in v5.12+.
>
> Sorry, I missed some things in previous review rounds:
>
> [...]
> > @@ -18,31 +18,37 @@ static int parse_build_id_buf(unsigned char *build_id,
> [...]
> >                 if (nhdr->n_type == BUILD_ID &&
> > -                   nhdr->n_namesz == sizeof("GNU") &&
> > -                   !strcmp((char *)(nhdr + 1), "GNU") &&
> > -                   nhdr->n_descsz > 0 &&
> > -                   nhdr->n_descsz <= BUILD_ID_SIZE_MAX) {
> > -                       memcpy(build_id,
> > -                              note_start + note_offs +
> > -                              ALIGN(sizeof("GNU"), 4) + sizeof(Elf32_Nhdr),
> > -                              nhdr->n_descsz);
> > -                       memset(build_id + nhdr->n_descsz, 0,
> > -                              BUILD_ID_SIZE_MAX - nhdr->n_descsz);
> > +                   name_sz == note_name_sz &&
> > +                   strcmp((char *)(nhdr + 1), note_name) == 0 &&
>
> Please change this to something like "memcmp((char *)(nhdr + 1),
> note_name, note_name_sz) == 0" to ensure that we can't run off the end
> of the page if there are no null bytes in the rest of the page.

I did switch this to strncmp() at some earlier point, but then
realized that there is no point because note_name is controlled by us
and will ensure there is a zero at byte (note_name_sz - 1). So I don't
think memcmp() buys us anything.

>
> [...]
> > @@ -90,8 +97,8 @@ static int get_build_id_32(const void *page_addr, unsigned char *build_id,
> >         for (i = 0; i < ehdr->e_phnum; ++i) {
>
> Please change this to "for (i = 0; i < phnum; ++i) {" like in the
> 64-bit version.

This did slip through, yep. I'll check with BPF maintainers if this
can be fixed up while applying or whether I should send another
revision.

>
> With these two changes applied:
>
> Reviewed-by: Jann Horn <jannh@google.com>