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

[Kees Cook]

On Thu, Dec 10, 2015 at 11:33 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Dec 10, 2015 at 07:16:11PM +0100, Willy Tarreau wrote:
>
>> > Is f_flags safe to write like this without holding a lock?
>>
>> Unfortunately I have no idea. I've seen places where it's written without
>> taking a lock such as in blkdev_open() and I don't think that this one is
>> called with a lock held.
>
> In any ->open() we obviously have nobody else able to find that struct file,
> let alone modify it, so there the damn thing is essentially caller-private
> and no locking is needed.

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.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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