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Date: Thu, 10 Dec 2015 20:27:49 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Kees Cook <keescook@chromium.org>
Cc: Willy Tarreau <w@1wt.eu>, Andrew Morton <akpm@linux-foundation.org>,
        Jan Kara <jack@suse.cz>, yalin wang <yalin.wang2010@gmail.com>,
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        Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v5] fs: clear file privilege bits when mmap writing
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On Thu, Dec 10, 2015 at 11:47:18AM -0800, Kees Cook wrote:

> In open, sure, but what about under mm/memory.c where we're trying to
> twiddle it from vma->file->f_flags as in my patch? That seemed like it
> would want atomic safety.

Sigh...  Again, I'm not at all convinced that this is the right approach,
but generally you need ->f_lock.  And in situations where the bit can
go only off->on, check it lockless, skip the whole thing entirely if it's
already set and grab the spinlock otherwise.