Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20200130064430.17198-1-walter-zh.wu@mediatek.com>
In-Reply-To: <20200130064430.17198-1-walter-zh.wu@mediatek.com>
From:   Alexander Potapenko <glider@google.com>
Date:   Thu, 30 Jan 2020 13:03:58 +0100
Message-ID: <CAG_fn=X_jSUJXD932z9oN5hBa--n3Qct4zrjzGaPtb2MwJye7A@mail.gmail.com>
Subject: Re: [PATCH v3] lib/stackdepot: Fix global out-of-bounds in stackdepot
To:     Walter Wu <walter-zh.wu@mediatek.com>
Cc:     Dmitry Vyukov <dvyukov@google.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Kate Stewart <kstewart@linuxfoundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org,
        wsd_upstream <wsd_upstream@mediatek.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, Jan 30, 2020 at 7:44 AM Walter Wu <walter-zh.wu@mediatek.com> wrote=
:

Hi Walter,

> If the depot_index =3D STACK_ALLOC_MAX_SLABS - 2 and next_slab_inited =3D=
 0,
> then it will cause array out-of-bounds access, so that we should modify
> the detection to avoid this array out-of-bounds bug.
>
> Assume depot_index =3D STACK_ALLOC_MAX_SLABS - 3
> Consider following call flow sequence:
>
> stack_depot_save()
>    depot_alloc_stack()
>       if (unlikely(depot_index + 1 >=3D STACK_ALLOC_MAX_SLABS)) //pass
>       depot_index++  //depot_index =3D STACK_ALLOC_MAX_SLABS - 2
>       if (depot_index + 1 < STACK_ALLOC_MAX_SLABS) //enter
>          smp_store_release(&next_slab_inited, 0); //next_slab_inited =3D =
0
>       init_stack_slab()
>          if (stack_slabs[depot_index] =3D=3D NULL) //enter and exit
>
> stack_depot_save()
>    depot_alloc_stack()
>       if (unlikely(depot_index + 1 >=3D STACK_ALLOC_MAX_SLABS)) //pass
>       depot_index++  //depot_index =3D STACK_ALLOC_MAX_SLABS - 1
>       init_stack_slab(&prealloc)
>          stack_slabs[depot_index + 1]  //here get global out-of-bounds
>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Matthias Brugger <matthias.bgg@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Alexander Potapenko <glider@google.com>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Kate Stewart <kstewart@linuxfoundation.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Kate Stewart <kstewart@linuxfoundation.org>
> Signed-off-by: Walter Wu <walter-zh.wu@mediatek.com>
> ---
> changes in v2:
> modify call flow sequence and preconditon
>
> changes in v3:
> add some reviewers
> ---
>  lib/stackdepot.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index ed717dd08ff3..7e8a15e41600 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -106,7 +106,7 @@ static struct stack_record *depot_alloc_stack(unsigne=
d long *entries, int size,
>         required_size =3D ALIGN(required_size, 1 << STACK_ALLOC_ALIGN);
>
>         if (unlikely(depot_offset + required_size > STACK_ALLOC_SIZE)) {
> -               if (unlikely(depot_index + 1 >=3D STACK_ALLOC_MAX_SLABS))=
 {
> +               if (unlikely(depot_index + 2 >=3D STACK_ALLOC_MAX_SLABS))=
 {

I don't think this is the right way to fix the problem.
You're basically throwing away the last element of stack_slabs[], as
we won't allocate anything from it.

How about we set |next_slab_inited| to 1 here:

diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index 2e7d2232ed3c..943a51eb746d 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -105,6 +105,8 @@ static bool init_stack_slab(void **prealloc)
                return true;
        if (stack_slabs[depot_index] =3D=3D NULL) {
                stack_slabs[depot_index] =3D *prealloc;
+               if (depot_index + 1 =3D=3D STACK_ALLOC_MAX_SLABS)
+                       smp_store_release(&next_slab_inited, 1);
        } else {
                stack_slabs[depot_index + 1] =3D *prealloc;
                /*

This will ensure we won't be preallocating pages once |depot_index|
reaches the last element, and we won't attempt to write those pages
anywhere either.

Could you please check if this fixes the problem for you?

>                         WARN_ONCE(1, "Stack depot reached limit capacity"=
);
>                         return NULL;
>                 }
> --
> 2.18.0


--=20
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Stra=C3=9Fe, 33
80636 M=C3=BCnchen

Gesch=C3=A4ftsf=C3=BChrer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg