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Date:   Tue, 3 Aug 2021 13:08:03 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Luigi Rizzo <lrizzo@google.com>, Jann Horn <jannh@google.com>
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>, linux-mm@kvack.org
Subject: Re: [PATCH] Add mmap_assert_locked() annotations to find_vma*()
Message-ID: <20210803160803.GG543798@ziepe.ca>
References: <20210731175341.3458608-1-lrizzo@google.com>
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On Sat, Jul 31, 2021 at 10:53:41AM -0700, Luigi Rizzo wrote:
> find_vma() and variants need protection when used.
> This patch adds mmap_assert_lock() calls in the functions.
> 
> To make sure the invariant is satisfied, we also need to add a
> mmap_read_loc() around the get_user_pages_remote() call in
> get_arg_page(). The lock is not strictly necessary because the mm
> has been newly created, but the extra cost is limited because
> the same mutex was also acquired shortly before in __bprm_mm_init(),
> so it is hot and uncontended.
> 
> Signed-off-by: Luigi Rizzo <lrizzo@google.com>
>  fs/exec.c | 2 ++
>  mm/mmap.c | 2 ++
>  2 files changed, 4 insertions(+)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 38f63451b928..ac7603e985b4 100644
> +++ b/fs/exec.c
> @@ -217,8 +217,10 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos,
>  	 * We are doing an exec().  'current' is the process
>  	 * doing the exec and bprm->mm is the new process's mm.
>  	 */
> +	mmap_read_lock(bprm->mm);
>  	ret = get_user_pages_remote(bprm->mm, pos, 1, gup_flags,
>  			&page, NULL, NULL);
> +	mmap_read_unlock(bprm->mm);
>  	if (ret <= 0)
>  		return NULL;

Wasn't Jann Horn working on something like this too?

https://lore.kernel.org/linux-mm/20201016225713.1971256-1-jannh@google.com/

IIRC it was very tricky here, are you sure it is OK to obtain this lock
here?

I would much rather see Jann's complete solution be merged then
hacking at the exec problem on the side..

Jason