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

Eric W. Biederman
2020-03-06T05:11:23Z

Bernd Edlinger <bernd.edlinger@hotmail.de> 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@xmission.com>
>> ---
>> 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