[Libguestfs] [libguestfs-common PATCH 01/12] options: fix buffer overflow in get_keys() [CVE-2022-2211]

[Laszlo Ersek]

On 06/28/22 16:17, Richard W.M. Jones wrote:
> On Tue, Jun 28, 2022 at 01:49:04PM +0200, Laszlo Ersek wrote:
>> When calculating the greatest possible number of matching keys in
>> get_keys(), the current expression
>>
>>   MIN (1, ks->nr_keys)
>>
>> is wrong -- it will return at most 1.
>>
>> If all "nr_keys" keys match however, then we require "nr_keys" non-NULL
>> entries in the result array; in other words, we need
>>
>>   MAX (1, ks->nr_keys)
>>
>> (The comment just above the expression is correct; the code is wrong.)
>>
>> This buffer overflow is easiest to trigger in those guestfs tools that
>> parse the "--key" option in C; that is, with "OPTION_key". For example,
>> the command
>>
>> $ virt-cat $(seq -f '--key /dev/sda2:key:%g' 200) -d DOMAIN /no-such-file
>>
>> which passes 200 (different) passphrases for the LUKS-encrypted block
>> device "/dev/sda2", crashes with a SIGSEGV.
> 
> A slightly better reproducer is this, since it doesn't require you to
> have an encrypted guest around:
> 
> $ echo TEST | guestfish --keys-from-stdin -N part luks-format /dev/sda1 0
> $ virt-cat $(seq -f '--key /dev/sda1:key:%g' 200) -a test1.img /no-such-file
> Segmentation fault (core dumped)
> $ rm test1.img

I'll include this in the commit message.

> 
>> (
>>
>>   The buffer overflow is possible to trigger in OCaml-language tools as
>>   well; that is, those that call "create_standard_options" with
>>   ~key_opts:true.
>>
>>   Triggering the problem that way is less trivial. The reason is that when
>>   the OCaml tools parse the "--key" options, they de-duplicate the options
>>   first, based on the device identifier.
>>
>>   Thus, in theory, this de-duplication masks the issue, as (one would
>>   think) only one "--key" option could belong to a single device, and
>>   therefore the buffer overflow would not be triggered in practice.
>>
>>   This is not the case however: the de-duplication does not collapse keys
>>   that are provided for the same device, but use different identifier
>>   types (such as pathname of device node versus LUKS UUID) -- in that
>>   situation, two entries in the keystore will match the device, and the
>>   terminating NULL entry will not be present once get_keys() returns. In
>>   this scenario, we don't have an out-of-bounds write, but an
>>   out-of-bounds read, in decrypt_mountables() [options/decrypt.c].
>>
>>   There is *yet another* bug in get_keys() though that undoes the above
>>   "masking". The "uuid" parameter of get_keys() may be NULL (for example
>>   when the device to decrypt uses BitLocker and not LUKS). When this
>>   happens, get_keys() adds all keys in the keystore to the result array.
>>   Therefore, the out-of-bounds write is easy to trigger with
>>   OCaml-language tools as well, as long as we attempt to decrypt a
>>   BitLocker (not LUKS) device, and we pass the "--key" options with
>>   different device identifiers.
>>
>>   Subsequent patches in this series fix all of the above; this patch fixes
>>   the security bug.
>>
>> )
>>
>> Rather than replacing MIN with MAX, open-code the comparison, as we first
>> set "len" to 1 anyway.
>>
>> While at it, rework the NULL-termination such that the (len+1) addition
>> not go unchecked.
>>
>> Fixes: c10c8baedb88e7c2988a01b70fc5f81fa8e4885c
>> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1809453
>> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2100862
>> Signed-off-by: Laszlo Ersek <lersek at redhat.com>
>> ---
>>  options/keys.c | 16 +++++++++++-----
>>  1 file changed, 11 insertions(+), 5 deletions(-)
>>
>> diff --git a/options/keys.c b/options/keys.c
>> index 798315c2e95a..d27a7123e67e 100644
>> --- a/options/keys.c
>> +++ b/options/keys.c
>> @@ -126,21 +126,27 @@ read_first_line_from_file (const char *filename)
>>   * keystore, ask the user.
>>   */
>>  char **
>>  get_keys (struct key_store *ks, const char *device, const char *uuid)
>>  {
>> -  size_t i, j, len;
>> +  size_t i, j, nmemb;
>>    char **r;
>>    char *s;
>>  
>>    /* We know the returned list must have at least one element and not
>>     * more than ks->nr_keys.
>>     */
>> -  len = 1;
>> -  if (ks)
>> -    len = MIN (1, ks->nr_keys);
>> -  r = calloc (len+1, sizeof (char *));
>> +  nmemb = 1;
>> +  if (ks && ks->nr_keys > nmemb)
>> +    nmemb = ks->nr_keys;
>> +
>> +  /* make room for the terminating NULL */
>> +  if (nmemb == (size_t)-1)
>> +    error (EXIT_FAILURE, 0, _("size_t overflow"));
>> +  nmemb++;
>> +
>> +  r = calloc (nmemb, sizeof (char *));
>>    if (r == NULL)
>>      error (EXIT_FAILURE, errno, "calloc");
>>  
>>    j = 0;
> 
> Reviewed-by: Richard W.M. Jones <rjones at redhat.com>

Thanks!
Laszlo