6:50 a.m.
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(a)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(a)redhat.com>
Thanks!
Laszlo