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

[Dave Hansen]

On 05/07/2015 11:48 AM, Vlastimil Babka wrote:
> 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?

I guess it depends on the implementation.  I honestly don't know what
glibc's malloc does specifically.

>> 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...

Correct, is is not privileged.  The only way to "revoke" access would be
to disable the feature in CR4, in which case the keys wouldn't be
enforced either.

PKRU it saved/restored using xsave*/xrstor*, which require having the
FPU enabled.  But, you can still *use* them even if the FPU is not in
play.  So we can't use the FPU en/disable to help us, either. :(

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