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

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.

( 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&gt; --- 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;