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

[Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>]

On 12.03.2020 15:24, Eric W. Biederman wrote:
> Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx/
>>> Link: https://lore.kernel.org/lkml/AM6PR03MB5170B06F3A2B75EFB98D071AE4E60@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
>>> Link: https://lore.kernel.org/linux-fsdevel/20161102181806.GB1112@xxxxxxxxxx/
>>> Link: https://lore.kernel.org/lkml/20160923095031.GA14923@xxxxxxxxxx/
>>> Link: https://lore.kernel.org/lkml/20170213141452.GA30203@xxxxxxxxxx/
>>> 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@xxxxxxxxxxxx>
>>> ---
>>>  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:

[   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.