Re: [PATCH v8 14/18] luks2: Better error handling when setting up the cryptodisk

[Daniel Kiper <dkiper@net-space.pl>]

On Fri, Dec 11, 2020 at 07:10:18PM -0600, Glenn Washburn wrote:
> On Thu, 10 Dec 2020 17:07:07 +0100
> Daniel Kiper <dkiper@net-space.pl> wrote:
>
> > On Tue, Dec 08, 2020 at 04:45:45PM -0600, Glenn Washburn wrote:
> > > First, check to make sure that source disk has a known size. If
> > > not, print debug message and return error. There are 4 cases where
> > > GRUB_DISK_SIZE_UNKNOWN is set (biosdisk, obdisk, ofdisk, and
> > > uboot), and in all those cases processing continues. So this is
> > > probably a bit conservative. However, 3 of the cases seem
> > > pathological, and the other, biosdisk, happens when booting from a
> > > cd. Since I doubt booting from a LUKS2 volume on a cd is a big use
> > > case, we'll error until someone complains.
> > >
> > > Do some sanity checking on data coming from the luks header. If
> > > segment.size is "dynamic", verify that the offset is not past the
> > > end of disk. Otherwise, check for errors from grub_strtoull when
> > > converting segment size from string. If a GRUB_ERR_BAD_NUMBER error
> > > was returned, then the string was not a valid parsable number, so
> > > skip the key. If GRUB_ERR_OUT_OF_RANGE was returned, then there was
> > > an overflow in converting to a 64-bit unsigned integer. So this
> > > could be a very large disk (perhaps large raid array). In this
> > > case, we want to continue to try to use this key, but only allow
> > > access up to the end of the source disk.
> > >
> > > Signed-off-by: Glenn Washburn <development@efficientek.com>
> > > ---
> > >  grub-core/disk/luks2.c | 84
> > > ++++++++++++++++++++++++++++++++++++++++-- include/grub/disk.h    |
> > > 17 +++++++++ 2 files changed, 98 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/grub-core/disk/luks2.c b/grub-core/disk/luks2.c
> > > index 9abcb1c94..8cb11e899 100644
> > > --- a/grub-core/disk/luks2.c
> > > +++ b/grub-core/disk/luks2.c
> > > @@ -600,12 +600,26 @@ luks2_recover_key (grub_disk_t source,
> > >        goto err;
> > >      }
> > >
> > > +  if (source->total_sectors == GRUB_DISK_SIZE_UNKNOWN)
> > > +    {
> > > +      /* FIXME: Allow use of source disk, and maybe cause errors
> > > in read. */
> > > +      grub_dprintf ("luks2", "Source disk %s has an unknown size, "
> > > +			     "conservatively returning error\n",
> > > source->name);
> > > +      ret = grub_error (GRUB_ERR_BUG, "Unknown size of luks2
> > > source device");
> > > +      goto err;
> > > +    }
> > > +
> > >    /* Try all keyslot */
> > >    for (json_idx = 0; json_idx < size; json_idx++)
> > >      {
> > > +      typeof(source->total_sectors) max_crypt_sectors = 0;
> > > +
> > > +      grub_errno = GRUB_ERR_NONE;
> > >        ret = luks2_get_keyslot (&keyslot, &digest, &segment, json,
> > > json_idx); if (ret)
> > >  	goto err;
> > > +      if (grub_errno != GRUB_ERR_NONE)
> > > +	  grub_dprintf ("luks2", "Ignoring unhandled error %d from
> > > luks2_get_keyslot\n", grub_errno);
> > >
> > >        if (keyslot.priority == 0)
> > >  	{
> > > @@ -619,11 +633,75 @@ luks2_recover_key (grub_disk_t source,
> > >        crypt->offset_sectors = grub_divmod64 (segment.offset,
> > > segment.sector_size, NULL); crypt->log_sector_size = sizeof
> > > (unsigned int) * 8
> > >  		- __builtin_clz ((unsigned int)
> > > segment.sector_size) - 1;
> > > +      /* Set to the source disk size, which is the maximum we
> > > allow. */
> > > +      max_crypt_sectors = grub_disk_convert_sector(source,
> > > +
> > > source->total_sectors,
> > > +
> > > crypt->log_sector_size); +
> > > +      if (max_crypt_sectors < crypt->offset_sectors)
> > > +	{
> > > +	  grub_dprintf ("luks2", "Segment \"%"PRIuGRUB_UINT64_T"\"
> > > has offset"
> > > +				 " %"PRIuGRUB_UINT64_T" which is
> > > greater than"
> > > +				 " source disk size
> > > %"PRIuGRUB_UINT64_T","
> > > +				 " skipping\n",
> > > +				 segment.idx,
> > > crypt->offset_sectors,
> > > +				 max_crypt_sectors);
> > > +	  continue;
> > > +	}
> > > +
> > >        if (grub_strcmp (segment.size, "dynamic") == 0)
> > > -	crypt->total_sectors = (grub_disk_get_size (source) >>
> > > (crypt->log_sector_size - source->log_sector_size))
> > > -			       - crypt->offset_sectors;
> > > +	crypt->total_sectors = max_crypt_sectors -
> > > crypt->offset_sectors; else
> > > -	crypt->total_sectors = grub_strtoull (segment.size, NULL,
> > > 10) >> crypt->log_sector_size;
> > > +	{
> > > +	  grub_errno = GRUB_ERR_NONE;
> > > +	  /* Convert segment.size to sectors, rounding up to
> > > nearest sector */
> > > +	  crypt->total_sectors = grub_strtoull (segment.size,
> > > NULL, 10);
> > > +	  crypt->total_sectors = ALIGN_UP (crypt->total_sectors,
> > > +					   1 <<
> > > crypt->log_sector_size);
> > > +	  crypt->total_sectors >>= crypt->log_sector_size;
> > > +
> > > +	  if (grub_errno == GRUB_ERR_NONE)
> > > +	    ;
> > > +	  else if (grub_errno == GRUB_ERR_BAD_NUMBER)
> > > +	    {
> > > +	      grub_dprintf ("luks2", "Segment
> > > \"%"PRIuGRUB_UINT64_T"\" size"
> > > +				     " \"%s\" is not a parsable
> > > number\n",
> > > +				     segment.idx, segment.size);
> > > +	      continue;
> > > +	    }
> > > +	  else if (grub_errno == GRUB_ERR_OUT_OF_RANGE)
> > > +	    {
> > > +	      /*
> > > +	       * There was an overflow in parsing segment.size, so
> > > disk must
> > > +	       * be very large or the string is incorrect.
> > > +	       */
> > > +	      grub_dprintf ("luks2", "Segment
> > > \"%"PRIuGRUB_UINT64_T"\" size"
> > > +				     " %s overflowed 64-bit
> > > unsigned integer,"
> > > +				     " the end of the crypto
> > > device will be"
> > > +				     " inaccessible\n",
> > > +				     segment.idx, segment.size);
> > > +	      if (crypt->total_sectors > max_crypt_sectors)
> >
> > I think this if is bogus. You should clamp crypt->total_sectors
> > without any checks here.
>
> Actually, I wouldn't call this a clamp because in the overflow case
> crypt->total_sectors always equals 0.  I just realized this, and its
> because grub_strtoull will return 2^64-1 thus causing the following
> ALIGN_UP to overflow returning 0.  Suffice to say that's not what we
> want. My original intent was what happened before the ALIGN_UP code was
> introduced, which would ALIGN_DOWN.

Ugh... Right. That is why I still think you should do further calculations
on value returned from grub_strtoull() if grub_errno == GRUB_ERR_NONE only.
I understand that then crypt->total_sectors does not contain total sectors
for a while but you can rectify this a bit by putting short comment
before grub_strtoull() call.

> Here's an example illustrating why I wanted and still think the intent
> of this check is reasonable. Suppose we have a disk size 2^67 bytes
> with 512 byte (2^9) sized sectors. It will have 2^58 sectors. Further
> suppose, there is a LUKS volume with size 2^65 bytes starting at the
> beginning and a sector size of 4096 bytes (2^12). This will cause an
> overflow, so grub_strtoull will return 2^64-1, this should have us set
> crypt->total_sectors to 2^52-1. Since we don't know how much the
> overflow is (1 byte or 1 terabyte), we don't know how many more sectors
> til the end of the LUKS encrypted area. In this case max_crypt_sectors
> will be 2^(58+9-12) => 2^57 sectors.  So here we see that
> crypt->total_sectors < max_crypt_sectors in the overflow case.  If we
> do as you suggest crypt->total_sectors will be set to 2^57, and thus
> it will be valid to read past the end of the encrypted data (ie. block
> 2^56 of the 4k sector crypt will be a sector starting at byte 2^68,
> which is more than the 2^65 byte size volume).
>
> On the one hand, I like your suggestion because it allows reading all
> possible encrypted data, at the cost of reading, decrypting, and
> returning non-encrypted data (ie random garbage).  While keeping the
> check, will prevent returning garbage at the cost of not allowing
> access to all encrypted sectors. I think we should keep the check and
> document a known limitation of 2^64 byte maximum sized LUKS volumes.
> And that larger sized volumes can be read only up to byte 2^64.

If think the code should look like this:

  /*
   * ...a comment saying what crypt->total_sectors contains
   * and why LUKS2 volumes larger than 2^64 does not work...
   */
  crypt->total_sectors = grub_strtoull (segment.size,  NULL, 10);

  if (grub_errno == GRUB_ERR_NONE)
    {
      crypt->total_sectors = ALIGN_UP (crypt->total_sectors, 1 << crypt->log_sector_size);
      crypt->total_sectors >>= crypt->log_sector_size;
    }
  else if (grub_errno == GRUB_ERR_BAD_NUMBER)
    {
      ...
      continue;
    }

  if (grub_errno == GRUB_ERR_OUT_OF_RANGE ||
      !crypt->total_sectors ||
      crypt->total_sectors > max_crypt_sectors)
    {
      ...
      continue;
      /*
       * Yes, I think we should not guess crypt->total_sectors
       * value and always fail. It seems safer.
       */
    }

Daniel