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

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Date:	Wed, 9 Dec 2015 17:21:30 -0800
From:	Christoph Hellwig <hch@...radead.org>
To:	Kees Cook <keescook@...omium.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-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] fs: clear file privilege bits when mmap writing

> 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.
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