Re: [PATCH v6 2/2] cryptodisk: Allows UUIDs to be compared in a dash-insensitive manner

[Glenn Washburn]

On Mon, 29 Aug 2022 07:38:24 +0200
Patrick Steinhardt <ps@pks.im> wrote:

> On Fri, Aug 19, 2022 at 06:06:15PM -0500, Glenn Washburn wrote:
> > A user can now specify UUID strings with dashes, instead of having to remove
> > dashes. This is backwards-compatability preserving and also fixes a source
> > of user confusion over the inconsistency with how UUIDs are specified
> > between file system UUIDs and cryptomount UUIDs. Since cryptsetup, the
> > reference implementation for LUKS, displays and generates UUIDs with dashes
> > there has been additional confusion when using the UUID strings from
> > cryptsetup as exact input into GRUB does not find the expected cryptodisk.
> > 
> > A new function grub_uuidcasecmp is added that is general enough to be used
> > other places where UUIDs are being compared.
> > 
> > Signed-off-by: Glenn Washburn <development@efficientek.com>
> > ---
> >  grub-core/disk/cryptodisk.c |  4 ++--
> >  grub-core/disk/geli.c       |  2 +-
> >  grub-core/disk/luks.c       | 21 ++++-----------------
> >  grub-core/disk/luks2.c      | 15 ++++-----------
> >  include/grub/misc.h         | 32 ++++++++++++++++++++++++++++++++
> >  5 files changed, 43 insertions(+), 31 deletions(-)
> > 
> > diff --git a/grub-core/disk/cryptodisk.c b/grub-core/disk/cryptodisk.c
> > index e89430812..a4cd8445f 100644
> > --- a/grub-core/disk/cryptodisk.c
> > +++ b/grub-core/disk/cryptodisk.c
> > @@ -702,7 +702,7 @@ grub_cryptodisk_open (const char *name, grub_disk_t 
> > disk)
> >    if (grub_memcmp (name, "cryptouuid/", sizeof ("cryptouuid/") - 1) == 0)
> >      {
> >        for (dev = cryptodisk_list; dev != NULL; dev = dev->next)
> > -   if (grub_strcasecmp (name + sizeof ("cryptouuid/") - 1, dev->uuid) == 0)
> > +   if (grub_uuidcasecmp (name + sizeof ("cryptouuid/") - 1, dev->uuid, 
> > sizeof (dev->uuid)) == 0)
> >       break;
> >      }
> >    else
> > @@ -929,7 +929,7 @@ grub_cryptodisk_get_by_uuid (const char *uuid)
> >  {
> >    grub_cryptodisk_t dev;
> >    for (dev = cryptodisk_list; dev != NULL; dev = dev->next)
> > -    if (grub_strcasecmp (dev->uuid, uuid) == 0)
> > +    if (grub_uuidcasecmp (dev->uuid, uuid, sizeof (dev->uuid)) == 0)
> >        return dev;
> >    return NULL;
> >  }
> > diff --git a/grub-core/disk/geli.c b/grub-core/disk/geli.c
> > index e190066f9..722910d64 100644
> > --- a/grub-core/disk/geli.c
> > +++ b/grub-core/disk/geli.c
> > @@ -305,7 +305,7 @@ geli_scan (grub_disk_t disk, grub_cryptomount_args_t 
> > cargs)
> >        return NULL;
> >      }
> >  
> > -  if (cargs->search_uuid != NULL && grub_strcasecmp (cargs->search_uuid, 
> > uuid) != 0)
> > +  if (cargs->search_uuid != NULL && grub_uuidcasecmp (cargs->search_uuid, 
> > uuid, sizeof (uuid)) != 0)
> >      {
> >        grub_dprintf ("geli", "%s != %s\n", uuid, cargs->search_uuid);
> >        return NULL;
> > diff --git a/grub-core/disk/luks.c b/grub-core/disk/luks.c
> > index 7f837d52c..9e1e6a5d9 100644
> > --- a/grub-core/disk/luks.c
> > +++ b/grub-core/disk/luks.c
> > @@ -66,10 +66,7 @@ static grub_cryptodisk_t
> >  luks_scan (grub_disk_t disk, grub_cryptomount_args_t cargs)
> >  {
> >    grub_cryptodisk_t newdev;
> > -  const char *iptr;
> >    struct grub_luks_phdr header;
> > -  char *optr;
> > -  char uuid[sizeof (header.uuid) + 1];
> >    char ciphername[sizeof (header.cipherName) + 1];
> >    char ciphermode[sizeof (header.cipherMode) + 1];
> >    char hashspec[sizeof (header.hashSpec) + 1];
> > @@ -92,19 +89,9 @@ luks_scan (grub_disk_t disk, grub_cryptomount_args_t 
> > cargs)
> >        || grub_be_to_cpu16 (header.version) != 1)
> >      return NULL;
> >  
> > -  grub_memset (uuid, 0, sizeof (uuid));
> > -  optr = uuid;
> > -  for (iptr = header.uuid; iptr < &header.uuid[ARRAY_SIZE (header.uuid)];
> > -       iptr++)
> > +  if (cargs->search_uuid != NULL && grub_uuidcasecmp (cargs->search_uuid, 
> > header.uuid, sizeof (header.uuid)) != 0)
> >      {
> > -      if (*iptr != '-')
> > -   *optr++ = *iptr;
> > -    }
> > -  *optr = 0;
> > -
> > -  if (cargs->search_uuid != NULL && grub_strcasecmp (cargs->search_uuid, 
> > uuid) != 0)
> > -    {
> > -      grub_dprintf ("luks", "%s != %s\n", uuid, cargs->search_uuid);
> > +      grub_dprintf ("luks", "%s != %s\n", header.uuid, cargs->search_uuid);
> >        return NULL;
> >      }
> 
> I haven't noticed this in my previous review round, but I think this is
> wrong because `header.uuid` is never NUL-terminated. It is explicitly 40
> characters long and read directly from disk, so there wouldn't be any
> room for it to be NUL-terminated.

Why not? UUIDs are 32 hex characters with typically 4 dashes, that
makes 37 bytes needed including the NULL byte. In fact, I believe that
cryptsetup always creates the uuid field NULL terminated. Also, the
LUKS1 spec, and by extension LUKS2 spec, mandates NULL termination. The
uuid field is specified as "char[]" and "char[], a string stored as null
terminated sequence of 8-bit characters7".

> 
> So we'd rather have to:
> 
>     grub_dprintf ("luks2", "%.*s != %s\n", header.uuid, sizeof (header.uuid),
>                   cargs->search_uuid);
> 
> Sorry for not catching this in my first round.

This seems reasonable for what should be a very uncommon and non-spec
compliant case of having a uuid field with no NULL byte. I originally
had a patch before this that explicitly put a NULL byte at the end of
header.uuid so this problem didn't exist. I dropped it because it was
generally redundant as pointed out by Daniel, but I didn't take this
case into account. Perhaps its worth merging that with this one for
clarity.

> 
> >  
> > @@ -123,7 +110,7 @@ luks_scan (grub_disk_t disk, grub_cryptomount_args_t 
> > cargs)
> >    newdev->source_disk = NULL;
> >    newdev->log_sector_size = GRUB_LUKS1_LOG_SECTOR_SIZE;
> >    newdev->total_sectors = grub_disk_native_sectors (disk) - 
> > newdev->offset_sectors;
> > -  grub_memcpy (newdev->uuid, uuid, sizeof (uuid));
> > +  grub_memcpy (newdev->uuid, header.uuid, sizeof (header.uuid));
> >    newdev->modname = "luks";
> 
> This should be fine though because `newdev->uuid` is initialized to
> all-zeroes.
> 
> >    /* Configure the hash used for the AF splitter and HMAC.  */
> > @@ -143,7 +130,7 @@ luks_scan (grub_disk_t disk, grub_cryptomount_args_t 
> > cargs)
> >        return NULL;
> >      }
> >  
> > -  COMPILE_TIME_ASSERT (sizeof (newdev->uuid) >= sizeof (uuid));
> > +  COMPILE_TIME_ASSERT (sizeof (newdev->uuid) >= sizeof (header.uuid));
> >    return newdev;
> >  }
> 
> This has an off-by-one bug now though: `sizeof(uuid)` is not the same as
> `sizeof(header.uuid)`, but instead it was `sizeof(header.uuid)+1` to
> account for the trailing NUL-byte. So we have to make this:
> 
>     COMPILE_TIME_ASSERT (sizeof (newdev->uuid) > sizeof (header.uuid));

I don't think this is needed per above.

Please let me know if I've missed something and thanks for being
cautious. 

Glenn

> 
> > diff --git a/grub-core/disk/luks2.c b/grub-core/disk/luks2.c
> > index 5b3b36c8a..1174c4c2b 100644
> > --- a/grub-core/disk/luks2.c
> > +++ b/grub-core/disk/luks2.c
> > @@ -350,8 +350,6 @@ luks2_scan (grub_disk_t disk, grub_cryptomount_args_t 
> > cargs)
> >  {
> >    grub_cryptodisk_t cryptodisk;
> >    grub_luks2_header_t header;
> > -  char uuid[sizeof (header.uuid) + 1];
> > -  grub_size_t i, j;
> >  
> >    if (cargs->check_boot)
> >      return NULL;
> > @@ -362,14 +360,9 @@ luks2_scan (grub_disk_t disk, grub_cryptomount_args_t 
> > cargs)
> >        return NULL;
> >      }
> >  
> > -  for (i = 0, j = 0; i < sizeof (header.uuid); i++)
> > -    if (header.uuid[i] != '-')
> > -      uuid[j++] = header.uuid[i];
> > -  uuid[j] = '\0';
> > -
> > -  if (cargs->search_uuid != NULL && grub_strcasecmp (cargs->search_uuid, 
> > uuid) != 0)
> > +  if (cargs->search_uuid != NULL && grub_uuidcasecmp (cargs->search_uuid, 
> > header.uuid, sizeof (header.uuid)) != 0)
> >      {
> > -      grub_dprintf ("luks2", "%s != %s\n", uuid, cargs->search_uuid);
> > +      grub_dprintf ("luks2", "%s != %s\n", header.uuid, 
> > cargs->search_uuid);
> >        return NULL;
> >      }
> 
> Same here.
> 
> > @@ -377,8 +370,8 @@ luks2_scan (grub_disk_t disk, grub_cryptomount_args_t 
> > cargs)
> >    if (!cryptodisk)
> >      return NULL;
> >  
> > -  COMPILE_TIME_ASSERT (sizeof (cryptodisk->uuid) >= sizeof (uuid));
> > -  grub_memcpy (cryptodisk->uuid, uuid, sizeof (uuid));
> > +  COMPILE_TIME_ASSERT (sizeof (cryptodisk->uuid) >= sizeof (header.uuid));
> > +  grub_memcpy (cryptodisk->uuid, header.uuid, sizeof (header.uuid));
> 
> And here.
> 
> Patrick
> 
> >    cryptodisk->modname = "luks2";
> >    return cryptodisk;
> > diff --git a/include/grub/misc.h b/include/grub/misc.h
> > index 1c25c1767..76c5c7992 100644
> > --- a/include/grub/misc.h
> > +++ b/include/grub/misc.h
> > @@ -244,6 +244,38 @@ grub_strncasecmp (const char *s1, const char *s2, 
> > grub_size_t n)
> >      - (int) grub_tolower ((grub_uint8_t) *s2);
> >  }
> >  
> > +/*
> > + * Do a case insensitive compare of two UUID strings by ignoring all 
> > dashes.
> > + * Note that the parameter n, is the number of significant characters to
> > + * compare, where significant characters are any except the dash.
> > + */
> > +static inline int
> > +grub_uuidcasecmp (const char *uuid1, const char *uuid2, grub_size_t n)
> > +{
> > +  if (n == 0)
> > +    return 0;
> > +
> > +  while (*uuid1 && *uuid2 && --n)
> > +    {
> > +      /* Skip forward to non-dash on both UUIDs. */
> > +      while ('-' == *uuid1)
> > +        ++uuid1;
> > +
> > +      while ('-' == *uuid2)
> > +        ++uuid2;
> > +
> > +      if (grub_tolower ((grub_uint8_t) *uuid1)
> > +     != grub_tolower ((grub_uint8_t) *uuid2))
> > +   break;
> > +
> > +      uuid1++;
> > +      uuid2++;
> > +    }
> > +
> > +  return (int) grub_tolower ((grub_uint8_t) *uuid1)
> > +    - (int) grub_tolower ((grub_uint8_t) *uuid2);
> > +}
> > +
> >  /*
> >   * Note that these differ from the C standard's definitions of strtol,
> >   * strtoul(), and strtoull() by the addition of two const qualifiers on 
> > the end
> > -- 
> > 2.34.1
> >