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

[Bernd Edlinger]

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@xxxxxxxxxxxx>
>> ---
>>  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@xxxxxxxxxxxx>
>