Re: pkeys on POWER: Access rights not reset on execve

[Ram Pai <linuxram@us.ibm.com>]

On Sat, May 19, 2018 at 04:47:23PM -0700, Andy Lutomirski wrote: > On Sat, May 19, 2018 at 1:28 PM Ram Pai <linuxram@us.ibm.com> wrote:

...snip...
> 
> So is it possible for two threads to each call pkey_alloc() and end up with
> the same key?  If so, it seems entirely broken. 

No. Two threads cannot allocate the same key; just like x86. 

> If not, then how do you
> intend for a multithreaded application to usefully allocate a new key?
> Regardless, it seems like the current behavior on POWER is very difficult
> to work with.  Can you give an example of a use case for which POWER's
> behavior makes sense?
> 
> For the use cases I've imagined, POWER's behavior does not make sense.
>   x86's is not ideal but is still better.  Here are my two example use cases:
> 
> 1. A crypto library.  Suppose I'm writing a TLS-terminating server, and I
> want it to be resistant to Heartbleed-like bugs.  I could store my private
> keys protected by mprotect_key() and arrange for all threads and signal
> handlers to have PKRU/AMR values that prevent any access to the memory.
> When an explicit call is made to sign with the key, I would temporarily
> change PKRU/AMR to allow access, compute the signature, and change PKRU/AMR
> back.  On x86 right now, this works nicely.  On POWER, it doesn't, because
> any thread started before my pkey_alloc() call can access the protected
> memory, as can any signal handler.
> 
> 2. A database using mmap() (with persistent memory or otherwise).  It would
> be nice to be resistant to accidental corruption due to stray writes.  I
> would do more or less the same thing as (1), except that I would want
> threads that are not actively writing to the database to be able the
> protected memory.  On x86, I need to manually convince threads that may
> have been started before my pkey_alloc() call as well as signal handlers to
> update their PKRU values.  On POWER, as in example (1), the error goes the
> other direction -- if I fail to propagate the AMR bits to all threads,
> writes are not blocked.

I see the problem from an application's point of view, on powerpc.  If
the key allocated in one thread is not activated on all threads
(existing one and future one), than other threads will not be able
to modify the key's permissions. Hence they will not be able to control
access/write to pages to which the key is associated.

As Florian suggested, I should enable the key's bit in the UAMOR value
corresponding to existing threads, when a new key is allocated.

Now, looking at the implementation for x86, I see that sys_mpkey_alloc()
makes no attempt to modify anything of any other thread. How
does it manage to activate the key on any other thread? Is this
magic done by the hardware?

RP