Re: [PATCH 00/12] [RFC] x86: Memory Protection Keys

[Vlastimil Babka]

On 05/07/2015 08:09 PM, Dave Hansen wrote:
> On 05/07/2015 10:57 AM, Ingo Molnar wrote:
> > > > There are two new instructions (RDPKRU/WRPKRU) for reading and
> > > > writing to the new register.  The feature is only available in
> > > > 64-bit mode, even though there is theoretically space in the PAE
> > > > PTEs.  These permissions are enforced on data access only and have
> > > > no effect on instruction fetches.
> > So I'm wondering what the primary usecases are for this feature?
> > Could you outline applications/workloads/libraries that would
> > benefit from this?
>
> There are lots of things that folks would _like_ to mprotect(), but end
> up not being feasible because of the overhead of going and mucking with
> thousands of PTEs and shooting down remote TLBs every time you want to
> change protections.
>
> Data structures like logs or journals that are only written to in very
> limited code paths, but that you want to protect from "stray" writes.
>
> Maybe even a database where a query operation will never need to write
> to memory, but an insert would.  You could keep the data R/O during the
> entire operation except when an insert is actually in progress.  It
> narrows the window where data might be corrupted.  This becomes even
> more valuable if a stray write to memory is guaranteed to hit storage...
> like with persistent memory.
>
> Someone mentioned to me that valgrind does lots of mprotect()s and might
> benefit from this.
>
> We could keep heap metadata as R/O and only make it R/W inside of
> malloc() itself to catch corruption more quickly.

But that metadata is typically within the same page as the data itself 
(for small objects at least), no?

> More crazy ideas welcome. :)

Since you asked :) I wonder if the usefulness could be extended by 
making it possible for a thread to revoke its access to WRPKRU (it's not 
privileged, right?). Then I could imagine some extra security for 
sandbox/bytecode/JIT code so it doesn't interfere with the runtime. But 
since it doesn't block instruction fetches, then maybe it wouldn't make 
much difference...

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