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

[Kees Cook <keescook@chromium.org>]

On Wed, Dec 9, 2015 at 5:21 PM, Christoph Hellwig <hch@infradead.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@chromium.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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