Re: [Qemu-devel] [PATCH] block/vhdx: add check for truncated image files

[Kevin Wolf <kwolf@redhat.com>]

Am 02.09.2019 um 15:15 hat Peter Lieven geschrieben:
> Am 02.09.19 um 15:07 schrieb Kevin Wolf:
> > Am 29.08.2019 um 15:36 hat Peter Lieven geschrieben:
> > > qemu is currently not able to detect truncated vhdx image files.
> > > Add a basic check if all allocated blocks are reachable to vhdx_co_check.
> > > 
> > > Signed-off-by: Jan-Hendrik Frintrop <jhf@kamp.de>
> > > Signed-off-by: Peter Lieven <pl@kamp.de>
> > > ---
> > >   block/vhdx.c | 19 +++++++++++++++++++
> > >   1 file changed, 19 insertions(+)
> > > 
> > > diff --git a/block/vhdx.c b/block/vhdx.c
> > > index 6a09d0a55c..4382b1375d 100644
> > > --- a/block/vhdx.c
> > > +++ b/block/vhdx.c
> > > @@ -2068,10 +2068,29 @@ static int coroutine_fn vhdx_co_check(BlockDriverState *bs,
> > >                                         BdrvCheckMode fix)
> > >   {
> > >       BDRVVHDXState *s = bs->opaque;
> > > +    VHDXSectorInfo sinfo;
> > > +    int64_t file_size = bdrv_get_allocated_file_size(bs);
> > Don't you mean bdrv_getlength()?
> > 
> > bdrv_get_allocated_file_size() is only the allocated size, i.e. without
> > holes. So a higher offset may actually be present.
> 
> 
> Isn't bdrv_getlength the virtual disk size? I need to check if a block
> points to a location after EOF of the underlying physical file.

Yes, it would have to be bdrv_getlength(bs->file->bs), i.e. call it on
the protocol layer, not on the format layer.

> > > +    int64_t sector_num;
> > >       if (s->log_replayed_on_open) {
> > >           result->corruptions_fixed++;
> > >       }
> > > +
> > > +    for (sector_num = 0; sector_num < bs->total_sectors;
> > > +         sector_num += s->block_size / BDRV_SECTOR_SIZE) {
> > > +        int nb_sectors = MIN(bs->total_sectors - sector_num,
> > > +                             s->block_size / BDRV_SECTOR_SIZE);
> > > +        vhdx_block_translate(s, sector_num, nb_sectors, &sinfo);
> > > +        if ((s->bat[sinfo.bat_idx] & VHDX_BAT_STATE_BIT_MASK) ==
> > > +            PAYLOAD_BLOCK_FULLY_PRESENT) {
> > > +            if (sinfo.file_offset +
> > > +                sinfo.sectors_avail * BDRV_SECTOR_SIZE > file_size) {
> > Do we need to protect against integer overflows here? I think
> > sinfo.file_offset comes directly from the image file and might be
> > corrupted.
> > 
> > Or has it already been check somewhere?
> 
> 
> The headers are being checked in vhdx_open.  sinfo.file_offset +
> sinfo.sectors_avail * BDRV_SECTOR_SIZE is exactly what is being passed
> to bdrv_pread when reading from the image file.

Fair enough, though if I'm not missing anything, we only check that BAT
entries don't overlap with other regions, not that they aren't too high.
And vhdx_block_translate() doesn't seem to check for overflows either
before it sets sinfo->sectors_avail.

So maybe this is actually a bug that should be fixed in
vhdx_block_translate() so that normal accesses get the fix, too.

> > > +                /* block is past the end of file, image has been truncated. */
> > > +                result->corruptions++;
> > I think we should print an error message like other formats do, so that
> > the user knows which kind of corruption 'qemu-img check' found (include
> > the guest and host offset of the invalid block).
> 
> What would be the appropriate way to do this? There is no errp in the
> check funtion. Inclunde headers so that error_report() is available?

Yes, I think error_report() would be fine.

qcow2 even just uses fprintf(stderr, ...), but maybe that's something we
shouldn't imitate.

Kevin