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

From: Walter Wu
Date: Thu Jan 30 2020 - 21:05:53 EST


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
>
>
>