Re: [Qemu-devel] [Qemu-block] [PATCH] dmg: fix binary search

[Julio Faracco]

Hi Stefan,

I created a simple DMG file from MacOS to reproduce the problem:
https://bugs.launchpad.net/qemu/+bug/1809304

Em qua, 2 de jan de 2019 às 08:47, Stefan Hajnoczi <address@hidden>
escreveu:

> On Fri, Dec 21, 2018 at 09:58:03PM +0800, yuchenlin wrote:
> > There is a possible hang in original binary searsh implemtation. That is
> > if chunk1 = 4, chunk2 = 5, chunk3 = 4, and we go else case.
> >
> > The chunk1 will be still 4, and so on.
> >
> > Signed-off-by: yuchenlin <address@hidden>
> > ---
> >  block/dmg.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
>
> Do you have an example dmg file that triggers this bug?
>
> > diff --git a/block/dmg.c b/block/dmg.c
> > index 50e91aef6d..0e05702f5d 100644
> > --- a/block/dmg.c
> > +++ b/block/dmg.c
> > @@ -572,14 +572,14 @@ static inline uint32_t search_chunk(BDRVDMGState
> *s, uint64_t sector_num)
> >  {
> >      /* binary search */
> >      uint32_t chunk1 = 0, chunk2 = s->n_chunks, chunk3;
> > -    while (chunk1 != chunk2) {
> > +    while (chunk1 <= chunk2) {
> >          chunk3 = (chunk1 + chunk2) / 2;
> >          if (s->sectors[chunk3] > sector_num) {
> > -            chunk2 = chunk3;
> > +            chunk2 = chunk3 - 1;
>
> What if chunk1 = 0 and chunk2 = 1?  chunk3 = 0 and the new chunk2 value
> would be 0xffffffff so the loop continues with out-of-bounds sectors[]
> accesses!
>