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

[Patrick Steinhardt]

On Tue, Aug 30, 2022 at 03:12:36PM -0500, Glenn Washburn wrote:
> 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".

Indeed, you're right. I've read through the spec again and it matches
your explanation. That also makes my remaining points moot.

> > 
> > 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.

This is not as important anymore given that my initial point was moot.
But it may still be sensible as defennse against corrupted headers.

Anyway, with or without that change:

Reviewed-by: Patrick Steinhardt <ps@pks.im>

Thanks!

Patrick

> > 
> > >  
> > > @@ -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
> > >

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