Re: [RFC, PATCH] fix verify_write for 80386 support

[Brian Gerst <bgerst@quark.didntduck.org>]

Manfred Spraul wrote:
> [ 2nd part of the mail, after hitting the send button in the middle of 
> the sentence]
> 
> The solution is to perform the check for write protection in the actual 
> access functions, not during access_ok.
> 
> The attached patch does that:
> - remove the check from access_ok
> - redirect all user space write access into __copy_to_user_ll
> - within __copy_to_user_ll: use get_user_pages, and write directly to 
> the obtained kernel address.
> 
> This fixes all swapout & data corruption bugs, and even removes 26 lines 
> from the kernel.
> 
> What do you think? The alternative would be to drop support for real 
> 80386 cpus.
> 
> Not tested on a real 80386, I've just replace wp_works_ok with 
> !wp_works_ok.
> 
> -- 
>    Manfred
> 

Close, but there is still a race here.  You need to hold mmap_sem over 
the memcpy.  Another thread will expect the memory to not be written to 
after write-protecting it.

--
				Brian Gerst