Re: [PATCH v8 9/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC

[Kees Cook]

On Wed, Jun 25, 2014 at 11:20 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> On 06/25, Kees Cook wrote:
>>
>> On Wed, Jun 25, 2014 at 10:24 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>> >
>> > However, do_execve() takes cred_guard_mutex at the start in prepare_bprm_creds()
>> > and drops it in install_exec_creds(), so it should solve the problem?
>>
>> I can't tell yet. I'm still trying to understand the order of
>> operations here. It looks like de_thread() takes the sighand lock.
>> do_execve_common does:
>>
>> prepare_bprm_creds (takes cred_guard_mutex)
>> check_unsafe_exec (checks nnp to set LSM_UNSAFE_NO_NEW_PRIVS)
>> prepare_binprm (handles suid escalation, checks nnp separately)
>>     security_bprm_set_creds (checks LSM_UNSAFE_NO_NEW_PRIVS)
>> exec_binprm
>>     load_elf_binary
>>         flush_old_exec
>>             de_thread (takes and releases sighand->lock)
>>         install_exec_creds (releases cred_guard_mutex)
>
> Yes, and note that when cred_guard_mutex is dropped all other threads
> are already killed,
>
>> I don't see a way to use cred_guard_mutex during tsync (which holds
>> sighand->lock) without dead-locking. What were you considering here?
>
> Just take/drop current->signal->cred_guard_mutex along with ->siglock
> in seccomp_set_mode_filter() ? Unconditionally on depending on
> SECCOMP_FILTER_FLAG_TSYNC.

Yeah, this looks good. *whew* Testing it now, so far so good.

Thanks!

-Kees

-- 
Kees Cook
Chrome OS Security
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