Re: [PATCH v7 bpf-next 01/10] lib/buildid: harden build ID parsing logic

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

On Fri, Nov 1, 2024 at 6:54 AM Jiri Olsa <olsajiri@gmail.com> wrote:
>
> On Thu, Aug 29, 2024 at 10:42:23AM -0700, Andrii Nakryiko 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+.
> >
> > Cc: stable@vger.kernel.org
> > Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
> > Reviewed-by: Jann Horn <jannh@google.com>
> > Suggested-by: Andi Kleen <ak@linux.intel.com>
> > Fixes: bd7525dacd7e ("bpf: Move stack_map_get_build_id into lib")
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> > ---
> >  lib/buildid.c | 76 +++++++++++++++++++++++++++++----------------------
> >  1 file changed, 44 insertions(+), 32 deletions(-)
> >
> > diff --git a/lib/buildid.c b/lib/buildid.c
> > index e02b5507418b..26007cc99a38 100644
> > --- a/lib/buildid.c
> > +++ b/lib/buildid.c
> > @@ -18,31 +18,37 @@ static int parse_build_id_buf(unsigned char *build_id,
> >                             const void *note_start,
> >                             Elf32_Word note_size)
> >  {
> > -     Elf32_Word note_offs = 0, new_offs;
> > -
> > -     while (note_offs + sizeof(Elf32_Nhdr) < note_size) {
> > -             Elf32_Nhdr *nhdr = (Elf32_Nhdr *)(note_start + note_offs);
> > +     const char note_name[] = "GNU";
> > +     const size_t note_name_sz = sizeof(note_name);
> > +     u64 note_off = 0, new_off, name_sz, desc_sz;
> > +     const char *data;
> > +
> > +     while (note_off + sizeof(Elf32_Nhdr) < note_size &&
> > +            note_off + sizeof(Elf32_Nhdr) > note_off /* overflow */) {
> > +             Elf32_Nhdr *nhdr = (Elf32_Nhdr *)(note_start + note_off);
> > +
> > +             name_sz = READ_ONCE(nhdr->n_namesz);
> > +             desc_sz = READ_ONCE(nhdr->n_descsz);
> > +
> > +             new_off = note_off + sizeof(Elf32_Nhdr);
> > +             if (check_add_overflow(new_off, ALIGN(name_sz, 4), &new_off) ||
> > +                 check_add_overflow(new_off, ALIGN(desc_sz, 4), &new_off) ||
> > +                 new_off > note_size)
> > +                     break;
> >
> >               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 &&
> > +                 memcmp(nhdr + 1, note_name, note_name_sz) == 0 &&
> > +                 desc_sz > 0 && desc_sz <= BUILD_ID_SIZE_MAX) {
> > +                     data = note_start + note_off + ALIGN(note_name_sz, 4);
> > +                     memcpy(build_id, data, desc_sz);
> > +                     memset(build_id + desc_sz, 0, BUILD_ID_SIZE_MAX - desc_sz);
> >                       if (size)
> > -                             *size = nhdr->n_descsz;
> > +                             *size = desc_sz;
> >                       return 0;
> >               }
>
> hi,
> this fix is causing stable kernels to return wrong build id,
> the change below seems to fix that (based on 6.6 stable)
>
> if we agree on the fix I'll send it to all affected stable trees
>
> jirka
>
>
> ---
> The parse_build_id_buf does not account Elf32_Nhdr header size
> when getting the build id data pointer and returns wrong build
> id data as result.
>
> This is problem only stable trees that merged c83a80d8b84f fix,
> the upstream build id code was refactored and returns proper
> build id.
>
> Fixes: c83a80d8b84f ("lib/buildid: harden build ID parsing logic")
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  lib/buildid.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/buildid.c b/lib/buildid.c
> index d3bc3d0528d5..9fc46366597e 100644
> --- a/lib/buildid.c
> +++ b/lib/buildid.c
> @@ -40,7 +40,7 @@ static int parse_build_id_buf(unsigned char *build_id,
>                     name_sz == note_name_sz &&
>                     memcmp(nhdr + 1, note_name, note_name_sz) == 0 &&
>                     desc_sz > 0 && desc_sz <= BUILD_ID_SIZE_MAX) {
> -                       data = note_start + note_off + ALIGN(note_name_sz, 4);
> +                       data = note_start + note_off + sizeof(Elf32_Nhdr) + ALIGN(note_name_sz, 4);

ah, my screw up, sorry. LGTM

Acked-by: Andrii Nakryiko <andrii@kernel.org>

>                         memcpy(build_id, data, desc_sz);
>                         memset(build_id + desc_sz, 0, BUILD_ID_SIZE_MAX - desc_sz);
>                         if (size)
> --
> 2.47.0
>