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

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Message-ID: <554BAA68.6000508@sr71.net>
Date:	Thu, 07 May 2015 11:09:44 -0700
From:	Dave Hansen <dave@...1.net>
To:	Ingo Molnar <mingo@...nel.org>
CC:	linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH 00/12] [RFC] x86: Memory Protection Keys

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.

More crazy ideas welcome. :)
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