linux-kernel - Re: [PATCH 1/2] tomoyo: Convert get_user_pages*() to pin_user

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Date:   Sat, 7 Nov 2020 01:04:50 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Souptick Joarder <jrdr.linux@...il.com>, <takedakn@...data.co.jp>,
        <penguin-kernel@...ove.SAKURA.ne.jp>, <jmorris@...ei.org>,
        <serge@...lyn.com>
CC:     <linux-security-module@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH 1/2] tomoyo: Convert get_user_pages*() to
 pin_user_pages*()

On 11/7/20 12:24 AM, Souptick Joarder wrote:
> In 2019, we introduced pin_user_pages*() and now we are converting
> get_user_pages*() to the new API as appropriate. [1] & [2] could
> be referred for more information. This is case 5 as per document [1].

It turns out that Case 5 can be implemented via a better pattern, as long
as we're just dealing with a page at a time, briefly:

lock_page()
write to page's data
unlock_page()

...which neatly synchronizes with writeback and other fs activities.

I was going to track down the Case 5's and do that [1].

+CC Jan and Matthew, to keep us on the straight and narrow, just in case
I'm misunderstanding something.

[1] https://lore.kernel.org/r/e78fb7af-627b-ce80-275e-51f97f1f3168@nvidia.com

thanks,
-- 
John Hubbard
NVIDIA

> 
> [1] Documentation/core-api/pin_user_pages.rst
> 
> [2] "Explicit pinning of user-space pages":
>          https://lwn.net/Articles/807108/
> 
> Signed-off-by: Souptick Joarder <jrdr.linux@...il.com>
> Cc: John Hubbard <jhubbard@...dia.com>
> ---
>   security/tomoyo/domain.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/security/tomoyo/domain.c b/security/tomoyo/domain.c
> index dc4ecc0..bd748be 100644
> --- a/security/tomoyo/domain.c
> +++ b/security/tomoyo/domain.c
> @@ -914,7 +914,7 @@ bool tomoyo_dump_page(struct linux_binprm *bprm, unsigned long pos,
>   	 * (represented by bprm).  'current' is the process doing
>   	 * the execve().
>   	 */
> -	if (get_user_pages_remote(bprm->mm, pos, 1,
> +	if (pin_user_pages_remote(bprm->mm, pos, 1,
>   				FOLL_FORCE, &page, NULL, NULL) <= 0)
>   		return false;
>   #else
> @@ -936,7 +936,7 @@ bool tomoyo_dump_page(struct linux_binprm *bprm, unsigned long pos,
>   	}
>   	/* Same with put_arg_page(page) in fs/exec.c */
>   #ifdef CONFIG_MMU
> -	put_page(page);
> +	unpin_user_page(page);
>   #endif
>   	return true;
>   }
>