linux-kernel - Re: [PATCH v5] fs: clear file privilege bits when mmap writing

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Message-ID: <CAGXu5jL0Zv6mCoEw6pyZsgHjo8BdcF0B-xM_EkMtp7TRB94dKQ@mail.gmail.com>
Date:	Wed, 9 Dec 2015 19:25:26 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	yalin wang <yalin.wang2010@...il.com>,
	Willy Tarreau <w@....eu>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5] fs: clear file privilege bits when mmap writing

On Wed, Dec 9, 2015 at 5:21 PM, Christoph Hellwig <hch@...radead.org> wrote:
>> Changing the bits requires holding inode->i_mutex, so it cannot be done
>> during the page fault (due to mmap_sem being held during the fault). We
>> could do this during vm_mmap_pgoff, but that would need coverage in
>> mprotect as well, but to check for MAP_SHARED, we'd need to hold mmap_sem
>> again. We could clear at open() time, but it's possible things are
>> accidentally opening with O_RDWR and only reading. Better to clear on
>> close and error failures (i.e. an improvement over now, which is not
>> clearing at all).
>>
>> Instead, detect the need to clear the bits during the page fault, and
>> actually remove the bits during final fput. Since the file was open for
>> writing, it wouldn't have been possible to execute it yet.
>
>
>>
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>> ---
>> I think this is the best we can do; everything else is blocked by mmap_sem.
>
> It should be done at mmap time, before even taking mmap_sem.
>
> Adding a new field for this to strut file isn't really acceptable.

I already covered this: there's no way to handle the mprotect case --
checking for MAP_SHARED is under mmap_sem still.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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