Re: MPK: pkey_free and key reuse

[Florian Weimer <fweimer@redhat.com>]

On 11/23/2017 04:25 PM, Dave Hansen wrote:
> I don't see a way to do a broadcast PKRU update.  But, I'd love to be
> proven wrong, with code.

I could use the existing setxid broadcast code in glibc to update PKRU 
on all running threads upon a key allocation (before pkey_alloc returns 
to the application), but this won't work for the implicit protection key 
used for PROT_EXEC.  I don't see a good way to get its number, and to 
determine whether a particular mprotect call allocated it.  (We 
obviously don't want to do the broadcast on every mprotect call with 
PROT_EXEC, just in case.)

What's worse, the setxid broadcast is not async-signal-safe, so we can't 
use it from mprotect, which should better be async-signal-safe (I know 
that official, it's not, but it would still be problematic to change 
that IMHO).

(The setxid broadcast mechanism allows us to run a piece of code on all 
threads of the process.  We could look at %rip and see if the signal 
arrived during a pkey_set function call, and make sure that this call 
delivers the right result, by altering the task state before returning.)

Thanks,
Florian

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