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

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Message-ID: <40bd424d-6c4d-8b03-5d97-c572ca777b77@i-love.sakura.ne.jp>
Date:   Sun, 8 Nov 2020 13:12:09 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     John Hubbard <jhubbard@...dia.com>,
        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 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.