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

[Bernd Edlinger]

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;
>  
> @@ -1266,7 +1267,7 @@ int flush_old_exec(struct linux_binprm * bprm)
>  	 * Make sure we have a private signal table and that
>  	 * we are unassociated from the previous thread group.
>  	 */
> -	retval = de_thread(current);
> +	retval = de_thread(bprm, current);

can we get rid of passing current as parameter here?

Thanks
Bernd.

>  	if (retval)
>  		goto out;
>  
> @@ -1664,7 +1665,7 @@ int search_binary_handler(struct linux_binprm *bprm)
>  
>  		read_lock(&binfmt_lock);
>  		put_binfmt(fmt);
> -		if (retval < 0 && !bprm->mm) {
> +		if (retval < 0 && bprm->unrecoverable) {
>  			/* we got to flush_old_exec() and failed after it */
>  			read_unlock(&binfmt_lock);
>  			force_sigsegv(SIGSEGV);
> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
> index b40fc633f3be..12263115ce7a 100644
> --- a/include/linux/binfmts.h
> +++ b/include/linux/binfmts.h
> @@ -44,7 +44,12 @@ struct linux_binprm {
>  		 * exec has happened. Used to sanitize execution environment
>  		 * and to set AT_SECURE auxv for glibc.
>  		 */
> -		secureexec:1;
> +		secureexec:1,
> +		/*
> +		 * Set when changes have been made that prevent returning
> +		 * to userspace.
> +		 */
> +		unrecoverable:1;
>  #ifdef __alpha__
>  	unsigned int taso:1;
>  #endif
>