Re: [PATCH v2 2/5] exec: Factor unshare_sighand out of de_thread and call it separately

Bernd Edlinger
2020-03-10T20:34:10Z

On 3/10/20 9:29 PM, Kees Cook wrote:
> On Sun, Mar 08, 2020 at 04:36:17PM -0500, Eric W. Biederman wrote:
>>
>> This makes the code clearer and makes it easier to implement a mutex
>> that is not taken over any locations that may block indefinitely waiting
>> for userspace.
>>
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>> ---
>> fs/exec.c | 39 ++++++++++++++++++++++++++-------------
>> 1 file changed, 26 insertions(+), 13 deletions(-)
>>
>> diff --git a/fs/exec.c b/fs/exec.c
>> index c3f34791f2f0..ff74b9a74d34 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1194,6 +1194,23 @@ static int de_thread(struct task_struct *tsk)
>> flush_itimer_signals();
>> #endif
>
> Semi-related (existing behavior): in de_thread(), what keeps the thread
> group from changing? i.e.:
>
> if (thread_group_empty(tsk))
> goto no_thread_group;
>
> /*
> * Kill all other threads in the thread group.
> */
> spin_lock_irq(lock);
> ... kill other threads under lock ...
>
> Why is the thread_group_emtpy() test not under lock?
>

A new thread cannot created when only one thread is executing,
right?

>>
>> + BUG_ON(!thread_group_leader(tsk));
>> + return 0;
>> +
>> +killed:
>> + /* protects against exit_notify() and __exit_signal() */
>
> I wonder if include/linux/sched/task.h's definition of tasklist_lock
> should explicitly gain note about group_exit_task and notify_count,
> or, alternatively, signal.h's section on these fields should gain a
> comment? tasklist_lock is unmentioned in signal.h... :(
>
>> + read_lock(&tasklist_lock);
>> + sig->group_exit_task = NULL;
>> + sig->notify_count = 0;
>> + read_unlock(&tasklist_lock);
>> + return -EAGAIN;
>> +}
>> +
>> +
>> +static int unshare_sighand(struct task_struct *me)
>> +{
>> + struct sighand_struct *oldsighand = me->sighand;
>> +
>> if (refcount_read(&oldsighand->count) != 1) {
>> struct sighand_struct *newsighand;
>> /*
>> @@ -1210,23 +1227,13 @@ static int de_thread(struct task_struct *tsk)
>>
>> write_lock_irq(&tasklist_lock);
>> spin_lock(&oldsighand->siglock);
>> - rcu_assign_pointer(tsk->sighand, newsighand);
>> + rcu_assign_pointer(me->sighand, newsighand);
>> spin_unlock(&oldsighand->siglock);
>> write_unlock_irq(&tasklist_lock);
>>
>> __cleanup_sighand(oldsighand);
>> }
>> -
>> - BUG_ON(!thread_group_leader(tsk));
>> return 0;
>> -
>> -killed:
>> - /* protects against exit_notify() and __exit_signal() */
>> - read_lock(&tasklist_lock);
>> - sig->group_exit_task = NULL;
>> - sig->notify_count = 0;
>> - read_unlock(&tasklist_lock);
>> - return -EAGAIN;
>> }
>>
>> char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
>> @@ -1264,13 +1271,19 @@ int flush_old_exec(struct linux_binprm * bprm)
>> int retval;
>>
>> /*
>> - * Make sure we have a private signal table and that
>> - * we are unassociated from the previous thread group.
>> + * Make this the only thread in the thread group.
>> */
>> retval = de_thread(me);
>> if (retval)
>> goto out;
>>
>> + /*
>> + * Make the signal table private.
>> + */
>> + retval = unshare_sighand(me);
>> + if (retval)
>> + goto out;
>> +
>> /*
>> * Must be called _before_ exec_mmap() as bprm->mm is
>> * not visibile until then. This also enables the update
>> --
>> 2.25.0
>
> Otherwise, yes, sensible separation.
>
> Reviewed-by: Kees Cook <keescook@chromium.org>
>