Re: [PATCH 1/2] exec: Properly mark the point of no return

[Eric W. Biederman]

Bernd Edlinger <bernd.edlinger@xxxxxxxxxx> writes:

> On 3/5/20 10:15 PM, Eric W. Biederman wrote:
>> 
>> Add a flag binfmt->unrecoverable to mark when execution has gotten to
>> the point where it is impossible to return to userspace with the
>> calling process unchanged.
>> 
>> While techinically this state starts as soon as de_thread starts
>> killing threads, the only return path at that point is if there is a
>> fatal signal pending.  I have choosen instead to set unrecoverable
>> when the killing stops, and there are possibilities of failures other
>> than fatal signals.  In particular it is possible for the allocation
>> of a new sighand structure to fail.
>> 
>> Setting unrecoverable at this point has the benefit that other actions
>> can be taken after the other threads are all dead, and the
>> unrecoverable flag can double as a flag that those actions have been
>> taken.
>> 
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>> ---
>>  fs/exec.c               | 7 ++++---
>>  include/linux/binfmts.h | 7 ++++++-
>>  2 files changed, 10 insertions(+), 4 deletions(-)
>> 
>> diff --git a/fs/exec.c b/fs/exec.c
>> index db17be51b112..c243f9660d46 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1061,7 +1061,7 @@ static int exec_mmap(struct mm_struct *mm)
>>   * disturbing other processes.  (Other processes might share the signal
>>   * table via the CLONE_SIGHAND option to clone().)
>>   */
>> -static int de_thread(struct task_struct *tsk)
>> +static int de_thread(struct linux_binprm *bprm, struct task_struct *tsk)
>>  {
>>  	struct signal_struct *sig = tsk->signal;
>>  	struct sighand_struct *oldsighand = tsk->sighand;
>> @@ -1182,6 +1182,7 @@ static int de_thread(struct task_struct *tsk)
>>  		release_task(leader);
>>  	}
>>  
>> +	bprm->unrecoverable = true;
>>  	sig->group_exit_task = NULL;
>>  	sig->notify_count = 0;
>>  
>
> ah, sorry, 
>         if (thread_group_empty(tsk))
>                 goto no_thread_group;
> will skip this:
>
>         sig->group_exit_task = NULL;
>         sig->notify_count = 0;
>
> no_thread_group:
>         /* we have changed execution domain */
>         tsk->exit_signal = SIGCHLD;
>
> so I think the bprm->unrecoverable = true; should be here?

Absolutely.  Thank you very much.

This is why I try and keep things to one clear simple thing per patch so
silly thinkos like that can be caught.

Eric