Re: [PATCH v3] lib/stackdepot: Fix global out-of-bounds in stackdepot

[Walter Wu]

On Thu, 2020-01-30 at 13:03 +0100, Alexander Potapenko wrote:
> On Thu, Jan 30, 2020 at 7:44 AM Walter Wu <walter-zh.wu@xxxxxxxxxxxx> wrote:
> 
> Hi Walter,
> 
> > If the depot_index = STACK_ALLOC_MAX_SLABS - 2 and next_slab_inited = 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 = STACK_ALLOC_MAX_SLABS - 3
> > Consider following call flow sequence:
> >
> > stack_depot_save()
> >    depot_alloc_stack()
> >       if (unlikely(depot_index + 1 >= STACK_ALLOC_MAX_SLABS)) //pass
> >       depot_index++  //depot_index = STACK_ALLOC_MAX_SLABS - 2
> >       if (depot_index + 1 < STACK_ALLOC_MAX_SLABS) //enter
> >          smp_store_release(&next_slab_inited, 0); //next_slab_inited = 0
> >       init_stack_slab()
> >          if (stack_slabs[depot_index] == NULL) //enter and exit
> >
> > stack_depot_save()
> >    depot_alloc_stack()
> >       if (unlikely(depot_index + 1 >= STACK_ALLOC_MAX_SLABS)) //pass
> >       depot_index++  //depot_index = STACK_ALLOC_MAX_SLABS - 1
> >       init_stack_slab(&prealloc)
> >          stack_slabs[depot_index + 1]  //here get global out-of-bounds
> >
> > Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > Cc: Matthias Brugger <matthias.bgg@xxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> > Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > Cc: Kate Stewart <kstewart@xxxxxxxxxxxxxxxxxxx>
> > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Cc: Kate Stewart <kstewart@xxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Walter Wu <walter-zh.wu@xxxxxxxxxxxx>
> > ---
> > 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(unsigned long *entries, int size,
> >         required_size = ALIGN(required_size, 1 << STACK_ALLOC_ALIGN);
> >
> >         if (unlikely(depot_offset + required_size > STACK_ALLOC_SIZE)) {
> > -               if (unlikely(depot_index + 1 >= STACK_ALLOC_MAX_SLABS)) {
> > +               if (unlikely(depot_index + 2 >= 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.
> 
ok, I agree.
 
> 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] == NULL) {
>                 stack_slabs[depot_index] = *prealloc;
> +               if (depot_index + 1 == STACK_ALLOC_MAX_SLABS)
> +                       smp_store_release(&next_slab_inited, 1);
>         } else {
>                 stack_slabs[depot_index + 1] = *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?
> 
Consider above call flow sequence at first stack_depot_save(),
depot_index = STACK_ALLOC_MAX_SLABS - 2 before enter init_stack_slab(),
so the fixes should be below.

--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -83,6 +83,8 @@ static bool init_stack_slab(void **prealloc)
                return true;
        if (stack_slabs[depot_index] == NULL) {
                stack_slabs[depot_index] = *prealloc;
+               if (depot_index + 2 == STACK_ALLOC_MAX_SLABS)
+                       smp_store_release(&next_slab_inited, 1);
        } else {
                stack_slabs[depot_index + 1] = *prealloc;
                /*


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