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SubjectRe: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex
Jann Horn <jannh@google.com> writes:

> On Sun, Mar 8, 2020 at 10:41 PM Eric W. Biederman <ebiederm@xmission.com> 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.
> [...]
>> @@ -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;
>
> We're already holding the old mmap_sem, and now nest the
> exec_update_mutex inside it; but then while still holding the
> exec_update_mutex, we do mmput(), which can e.g. end up in ksm_exit(),
> which can do down_write(&mm->mmap_sem) from __ksm_exit(). So I think
> at least lockdep will be unhappy, and I'm not sure whether it's an
> actual problem or not.

Good point. I should double check the lock ordering here with mmap_sem.
It doesn't look like mmput takes mmap_sem, but still there might be a
lock inversion of some kind here. At least as far as lockdep is
concerned and we don't want anything like that.

Eric







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