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

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Message-ID: <1174ed3e-d33c-6de9-3873-b122013b295a@nvidia.com>
Date:   Sat, 7 Nov 2020 21:00:26 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Souptick Joarder <jrdr.linux@...il.com>
CC:     <linux-security-module@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>
Subject: Re: [PATCH 1/2] tomoyo: Convert get_user_pages*() to
 pin_user_pages*()

On 11/7/20 8:12 PM, Tetsuo Handa wrote:
> On 2020/11/08 11:17, John Hubbard wrote:
>>> Excuse me, but Documentation/core-api/pin_user_pages.rst says
>>> "CASE 5: Pinning in order to _write_ to the data within the page"
>>> while tomoyo_dump_page() is for "_read_ the data within the page".
>>> Do we want to convert to pin_user_pages_remote() or lock_page() ?
>>>
>>
>> Sorry, I missed the direction here, was too focused on the Case 5
>> aspect. Yes. Case 5 (which, again, I think we're about to re-document)
>> is only about *writing* to data within the page.
>>
>> So in this case, where it is just reading from the page, I think it's
>> already from a gup vs pup point of view.
>>
>> btw, it's not clear to me whether the current code is susceptible to any
>> sort of problem involving something writing to the page while it
>> is being dumped (I am curious). But changing from gup to pup wouldn't
>> fix that, if it were a problem. It a separate question from this patch.
> 
> The "struct page" tomoyo_dump_page() accesses is argv/envp arguments passed
> to execve() syscall. Therefore, these pages are not visible from threads
> except current thread, and thus there is no possibility that these pages
> are modified by other threads while current thread is reading.
> 

Perfect. So since I accidentally left out the word "correct" above (I meant
to write, "it's already correct"), let me be extra clear: Souptick, we
should just drop this patch.

thanks,
-- 
John Hubbard
NVIDIA