Re: [RFC PATCH 0/2] arm64: optional paranoid __{get,put}_user checks

[Will Deacon]

On Thu, Oct 26, 2017 at 10:09:40AM +0100, Mark Rutland wrote:
> Hi,
> 
> In Prague, Kees mentioned that it would be nice to have a mechanism to
> catch bad __{get,put}_user uses, such as the recent CVE-2017-5123 [1,2]
> issue with unsafe_put_user() in waitid().
> 
> These patches allow an optional access_ok() check to be dropped in
> arm64's __{get,put}_user() primitives. These will then BUG() if a bad
> user pointer is passed (which should only happen in the absence of an
> earlier access_ok() check).
> 
> The first patch rewrites the arm64 access_ok() check in C. This gives
> the compiler the visibility it needs to elide redundant access_ok()
> checks, so in the common case:
> 
>   get_user()
>     access_ok()
>     __get_user()
>       BUG_ON(!access_ok())
>       <uaccess asm>
> 
> ... the compiler can determine that the second access_ok() must return
> true, and can elide it along with the BUG_ON(), leaving:
> 
>   get_user()
>     access_ok()
>       __get_user()
>         <uaccess asm>
> 
> ... and thus this sanity check can have no cost in the common case.

Probably a stupid question, but why not just move the access_ok check
into __{get,put}_user and remove it from {get,put}_user? We can also
then move the uaccess_{enable,disable}_not_uao calls out from the __
variants so that we can implement user_access_{begin,end}.

Will