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

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Date:   Thu, 5 Mar 2020 22:56:01 +0000
From:   Bernd Edlinger <bernd.edlinger@...mail.de>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
CC:     Christian Brauner <christian.brauner@...ntu.com>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oleg Nesterov <oleg@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrei Vagin <avagin@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Yuyang Du <duyuyang@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Anshuman Khandual <anshuman.khandual@....com>,
        David Howells <dhowells@...hat.com>,
        James Morris <jamorris@...ux.microsoft.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian Kellner <christian@...lner.me>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH 1/2] exec: Properly mark the point of no return

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@...ssion.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
>