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

Bernd Edlinger
2020-03-05T22:56:06Z

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?

Bernd.
> @@ -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);
> 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
>