Problem Description:
In FreeBSD 15.0, the kernel structure used to represent user
credentials changed: previously the primary group ID was stored in
the first element of the array containing the list of supplementary
group IDs, whereas now the primary group ID is stored in a dedicated
field. This change was largely internal to the kernel and not
user-visible.
One function, group_is_primary(), was not properly updated as a
part of this transition. This function is used by mac_do to determine
the primary group ID of the credential after applying a transition
rule, used when the rule target does not explicitly specify a group.
As a result, with certain mac_do rules, it is possible for a
credential switch to incorrectly set the primary group ID to the
ID stored in the first element of the original credential's
supplementary group array. If the list of supplementary groups is
empty, this value will be 0, corresponding to the "wheel" group.
For example, a rule such as "uid=1001>uid=1002" can be abused to
set the primary group ID to 0 even if the process did not originally
belong to group 0.
Impact:
Certain mac_do rules can be abused to set a process' group ID
to 0. Note however, that the rule must apply to the caller in order
for the bug to be triggered, e.g., given the ruleset "uid=1001>uid=1002",
the user must have user ID 1001 in order to trigger the bug.
Further, logged-in users will in general have a non-empty supplementary
group list, in which case the bug can at worst be used to set the
credential's first supplementary group ID as its primary group ID.
Processes must explicitly remove themselves from all supplementary
groups, using the privileged setgroups(2) system call, in order to
exploit the bug to set 0 as the primary group ID.
Since membership in group 0 is often used to enable controlled
privilege escalation, the bug might be further exploitable to obtain
root privileges, depending on the system configuration. For instance,
a ruleset such as the following could be exploited by a process
running as user 1001 and with an empty supplementary group list:
"uid=1001>uid=1002;gid=0>uid=0".