Re: [Qemu-devel] [PATCH] fw_cfg: fix memory corruption when all fw_cfg slots are used

[Marcel Apfelbaum]

On 09/01/2018 15:09, Laszlo Ersek wrote:

Hi Laszlo,

I'll respond first to this mail' I'll take my time with the rest :)

> On 01/08/18 22:50, Marcel Apfelbaum wrote:
> > When all the fw_cfg slots are used, a write is made outside the
> > bounds of the fw_cfg files array as part of the sort algorithm.
> >
> > Fix it by avoiding an unnecessary array element move.
> > Fix also an assert while at it.
> >
> > Signed-off-by: Marcel Apfelbaum <address@hidden>
> > ---
> >   hw/nvram/fw_cfg.c | 6 ++++--
> >   1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
> > index 753ac0e4ea..4313484b21 100644
> > --- a/hw/nvram/fw_cfg.c
> > +++ b/hw/nvram/fw_cfg.c
> > @@ -784,7 +784,7 @@ void fw_cfg_add_file_callback(FWCfgState *s,  const char 
> > *filename,
> >        * index and "i - 1" is the one being copied from, thus the
> >        * unusual start and end in the for statement.
> >        */
> > -    for (i = count + 1; i > index; i--) {
> > +    for (i = count; i > index; i--) {
> >           s->files->f[i] = s->files->f[i - 1];
> >           s->files->f[i].select = cpu_to_be16(FW_CFG_FILE_FIRST + i);
> >           s->entries[0][FW_CFG_FILE_FIRST + i] =
>
> This hunk looks correct to me.

After my change or before?

I think I am right.
At this point we have "count" elements in the array.
That means the last element in the array is at arr[count - 1].
We want to make room for the new element at index, so we move
all the elements from index to index + 1.

The first element we should move is arr[count - 1] to arr[count].
But the code moved arr[count] to arr [count + 1].
This move is not needed.


 We currently have count elements in the
> array, so we cannot normally access the element *at* count. However, we
> are extending the array right now, therefore we can assign (store) the
> element at count (and then we'll increment count later). But accessing
> an element at (count+1) is wrong.
>
> > @@ -833,7 +833,6 @@ void *fw_cfg_modify_file(FWCfgState *s, const char 
> > *filename,
> >       assert(s->files);
> >   
> >       index = be32_to_cpu(s->files->count);
> > -    assert(index < fw_cfg_file_slots(s));
> >   
> >       for (i = 0; i < index; i++) {
> >           if (strcmp(filename, s->files->f[i].name) == 0) {
> > @@ -843,6 +842,9 @@ void *fw_cfg_modify_file(FWCfgState *s, const char 
> > *filename,
> >               return ptr;
> >           }
> >       }
> > +
> > +    assert(index < fw_cfg_file_slots(s));
> > +
> >       /* add new one */
> >       fw_cfg_add_file_callback(s, filename, NULL, NULL, NULL, data, len, true);
> >       return NULL;
>
> I think I agree with Marc-André here, when I say, replace the assert
> with a comment instead? (About the fact that fw_cfg_add_file_callback()
> will assert(), *if* we reach that far.)

Hmm, what should we add to the comment? "We lost, brace for impact :)"

My point, if we are going to abort, let's abort as early as we can.
But if is a consensus, I'll get rid of it.

Thanks,
Marcel

> Thanks
> Laszlo