Messages in this thread Patch in this message | | | From | Alexander Potapenko <> | Date | Fri, 31 Jan 2020 19:11:44 +0100 | Subject | Re: [PATCH v3] lib/stackdepot: Fix global out-of-bounds in stackdepot |
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On Fri, Jan 31, 2020 at 3:05 AM Walter Wu <walter-zh.wu@mediatek.com> wrote: > > 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@mediatek.com> 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@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(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)) {
This again means stack_slabs[STACK_ALLOC_MAX_SLABS - 2] gets initialized, but stack_slabs[STACK_ALLOC_MAX_SLABS - 1] doesn't, because we'll be bailing out from init_stack_slab() from now on. Does this patch actually fix the problem (do you have a reliable reproducer?) This addition of 2 is also counterintuitive, I don't think further readers will understand the logic behind it.
What if we just check that depot_index + 1 is a valid index before accessing it?
diff --git a/lib/stackdepot.c b/lib/stackdepot.c index 2e7d2232ed3c..c2e6ff18d716 100644 --- a/lib/stackdepot.c +++ b/lib/stackdepot.c @@ -106,7 +106,9 @@ static bool init_stack_slab(void **prealloc) if (stack_slabs[depot_index] == NULL) { stack_slabs[depot_index] = *prealloc; } else { - stack_slabs[depot_index + 1] = *prealloc; + /* If this is the last depot slab, do not touch the next one. */ + if (depot_index + 1 < STACK_ALLOC_MAX_SLABS) + stack_slabs[depot_index + 1] = *prealloc; /* * This smp_store_release pairs with smp_load_acquire() from * |next_slab_inited| above and in stack_depot_save().
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