Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex

Kirill Tkhai
2020-03-12T15:23:53Z

On 12.03.2020 17:38, Eric W. Biederman wrote:
> Kirill Tkhai <ktkhai@virtuozzo.com> writes:
>
>> On 12.03.2020 15:24, Eric W. Biederman wrote:
>>> Kirill Tkhai <ktkhai@virtuozzo.com> writes:
>>>
>>>> On 09.03.2020 00:38, Eric W. Biederman wrote:
>>>>>
>>>>> The cred_guard_mutex is problematic. The cred_guard_mutex is held
>>>>> over the userspace accesses as the arguments from userspace are read.
>>>>> The cred_guard_mutex is held of PTRACE_EVENT_EXIT as the the other
>>>>> threads are killed. The cred_guard_mutex is held over
>>>>> "put_user(0, tsk->clear_child_tid)" in exit_mm().
>>>>>
>>>>> Any of those can result in deadlock, as the cred_guard_mutex is held
>>>>> over a possible indefinite userspace waits for userspace.
>>>>>
>>>>> Add exec_update_mutex that is only held over exec updating process
>>>>> with the new contents of exec, so that code that needs not to be
>>>>> confused by exec changing the mm and the cred in ways that can not
>>>>> happen during ordinary execution of a process.
>>>>>
>>>>> The plan is to switch the users of cred_guard_mutex to
>>>>> exec_udpate_mutex one by one. This lets us move forward while still
>>>>> being careful and not introducing any regressions.
>>>>>
>>>>> Link: https://lore.kernel.org/lkml/20160921152946.GA24210@dhcp22.suse.cz/
>>>>> Link: https://lore.kernel.org/lkml/AM6PR03MB5170B06F3A2B75EFB98D071AE4E60@AM6PR03MB5170.eurprd03.prod.outlook.com/
>>>>> Link: https://lore.kernel.org/linux-fsdevel/20161102181806.GB1112@redhat.com/
>>>>> Link: https://lore.kernel.org/lkml/20160923095031.GA14923@redhat.com/
>>>>> Link: https://lore.kernel.org/lkml/20170213141452.GA30203@redhat.com/
>>>>> Ref: 45c1a159b85b ("Add PTRACE_O_TRACEVFORKDONE and PTRACE_O_TRACEEXIT facilities.")
>>>>> Ref: 456f17cd1a28 ("[PATCH] user-vm-unlock-2.5.31-A2")
>>>>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>>>>> ---
>>>>> fs/exec.c | 9 +++++++++
>>>>> include/linux/sched/signal.h | 9 ++++++++-
>>>>> init/init_task.c | 1 +
>>>>> kernel/fork.c | 1 +
>>>>> 4 files changed, 19 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/fs/exec.c b/fs/exec.c
>>>>> index d820a7272a76..ffeebb1f167b 100644
>>>>> --- a/fs/exec.c
>>>>> +++ b/fs/exec.c
>>>>> @@ -1014,6 +1014,7 @@ static int exec_mmap(struct mm_struct *mm)
>>>>> {
>>>>> struct task_struct *tsk;
>>>>> struct mm_struct *old_mm, *active_mm;
>>>>> + int ret;
>>>>>
>>>>> /* Notify parent that we're no longer interested in the old VM */
>>>>> tsk = current;
>>>>> @@ -1034,6 +1035,11 @@ static int exec_mmap(struct mm_struct *mm)
>>>>> return -EINTR;
>>>>> }
>>>>> }
>>>>> +
>>>>> + ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
>>>>> + if (ret)
>>>>> + return ret;
>>>>
>>>> You missed old_mm->mmap_sem unlock. See here:
>>>
>>> Duh. Thank you.
>>>
>>> I actually need to switch the lock ordering here, and I haven't yet
>>> because my son was sick yesterday.
>>
>> There is some fundamental problem with your patch, since the below fires in 100% cases
>> on current linux-next:
>
> Thank you.
>
> I have just backed this out of linux-next for now because it is clearly
> flawed.
>
> You make some good points about the recursion. I will go back to the
> drawing board and see what I can work out.
>
>
>> [ 22.838717] kernel BUG at fs/exec.c:1474!
>>
>> diff --git a/fs/exec.c b/fs/exec.c
>> index 47582cd97f86..0f77f8c94905 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1470,8 +1470,10 @@ static void free_bprm(struct linux_binprm *bprm)
>> {
>> free_arg_pages(bprm);
>> if (bprm->cred) {
>> - if (!bprm->mm)
>> + if (!bprm->mm) {
>> + BUG_ON(!mutex_is_locked(&current->signal->exec_update_mutex));
>> mutex_unlock(&current->signal->exec_update_mutex);
>> + }
>> mutex_unlock(&current->signal->cred_guard_mutex);
>> abort_creds(bprm->cred);
>> }
>> @@ -1521,6 +1523,7 @@ void install_exec_creds(struct linux_binprm *bprm)
>> * credentials; any time after this it may be unlocked.
>> */
>> security_bprm_committed_creds(bprm);
>> + BUG_ON(!mutex_is_locked(&current->signal->exec_update_mutex));
>> mutex_unlock(&current->signal->exec_update_mutex);
>> mutex_unlock(&current->signal->cred_guard_mutex);
>> }
>>
>> ---------------------------------------------------------------------------------------------
>>
>> First time the mutex is unlocked in:
>>
>> exec_binprm()->search_binary_handler()->.load_binary->install_exec_creds()
>>
>> Then exec_binprm()->search_binary_handler()->.load_binary->flush_old_exec() clears mm:
>>
>> bprm->mm = NULL;
>>
>> Second time the mutex is unlocked in free_bprm():
>>
>> if (bprm->cred) {
>> if (!bprm->mm)
>> mutex_unlock(&current->signal->exec_update_mutex);
>>
>> My opinion is we should not relay on side indicators like bprm->mm. Better you may
>> introduce struct linux_binprm::exec_update_mutex_is_locked. So the next person dealing
>> with this after you won't waste much time on diving into this. Also, if someone decides
>> to change the place, where bprm->mm is set into NULL, this person will bump into hell
>> of dependences between unrelated components like your newly introduced mutex.
>>
>> So, I'm strongly for *struct linux_binprm::exec_update_mutex_is_locked*, since this improves
>> modularity.
>
> Am I wrong or is that also a problem with cred_guard_mutex?

No, there is no a problem.

cred_guard_mutex is locked in a pair with bprm->cred = prepare_exec_creds() assignment.

cred_guard_mutex is unlocked in a pair with bprm->cred = NULL clearing (see install_exec_creds()).
Further free_bprm() skip unlock in case of bprm->cred is NULL.