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

[Kees Cook]

On Thu, Dec 10, 2015 at 12:27 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> 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,

I'm open to any suggestions. Every path I've tried has been ultimately
blocked by mmap_sem. :(

> 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.

And I can take f_lock safely under mmap_sem?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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