On 2024/01/29 13:10, Eric W. Biederman wrote:
>> @@ -1519,6 +1519,7 @@ static void free_bprm(struct linux_binprm *bprm)
>> }
>> free_arg_pages(bprm);
>> if (bprm->cred) {
>> + security_bprm_aborting_creds(bprm);
>> mutex_unlock(¤t->signal->cred_guard_mutex);
>> abort_creds(bprm->cred);
>
> Why isn't abort_creds calling security_free_cred enough here?
Because security_cred_free() from put_cred_rcu() is called from RCU callback
rather than from current thread doing execve().
TOMOYO wants to restore attributes of current thread doing execve().
> The fact that somewhere Tomoyo is modifying a credential that the rest
> of the kernel sees as read-only, and making it impossible to just
> restore that credential is very concerning from a maintenance
> perspective.
TOMOYO does not use "struct cred"->security.
TOMOYO uses only "struct task_struct"->security.
struct lsm_blob_sizes tomoyo_blob_sizes __ro_after_init = {
.lbs_task = sizeof(struct tomoyo_task),
};
TOMOYO uses security_task_alloc() for allocating "struct task_struct"->security,
security_task_free() for releasing "struct task_struct"->security,
security_bprm_check() for updating "struct task_struct"->security,
security_bprm_committed_creds() for erasing old "struct task_struct"->security,
security_bprm_aborting_creds() for restoring old "struct task_struct"->security.
Commit a6f76f23d297 ("CRED: Make execve() take advantage of copy-on-write
credentials") made TOMOYO impossible to do above. current->in_execve flag was a
hack for emulating security_bprm_aborting_creds() using security_prepare_creds().
> Can't Tomoyo simply allow reading of files that have __FMODE_EXEC
> set when allow_execve is set, without needing to perform a domain
> transition, and later back out that domain transition?
No. That does not match TOMOYO's design.
allow_execve keyword does not imply "allow opening that file for non-execve() purpose".
Also, performing a domain transition before execve() reaches point of no return is
the TOMOYO's design, but COW credentials does not allow such behavior.
