linux-kernel - Re: [PATCH v2 2/2] exec: Broadly lock nascent mm until setup_arg

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Date:   Wed, 7 Oct 2020 09:12:34 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Jann Horn <jannh@...gle.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Michel Lespinasse <walken@...gle.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        <linux-um@...ts.infradead.org>,
        "John Hubbard" <jhubbard@...dia.com>
Subject: Re: [PATCH v2 2/2] exec: Broadly lock nascent mm until
 setup_arg_pages()

On Wed, Oct 07, 2020 at 12:54:50AM +0200, Jann Horn wrote:
> @@ -1545,6 +1532,18 @@ void setup_new_exec(struct linux_binprm * bprm)
>  	me->mm->task_size = TASK_SIZE;
>  	mutex_unlock(&me->signal->exec_update_mutex);
>  	mutex_unlock(&me->signal->cred_guard_mutex);
> +
> +#ifndef CONFIG_MMU
> +	/*
> +	 * On MMU, setup_arg_pages() wants to access bprm->vma after this point,
> +	 * so we can't drop the mmap lock yet.
> +	 * On !MMU, we have neither setup_arg_pages() nor bprm->vma, so we
> +	 * should drop the lock here.
> +	 */
> +	mmap_write_unlock(bprm->mm);
> +	mmput(bprm->mm);
> +	bprm->mm = NULL;
> +#endif
>  }

It looks like this could this be a

  if (!IS_ENABLED(CONFIG_MMU))

This all seems nice, more locking points were removed than added at
least

Jason