linux-hardening - Re: [RESEND PATCH v4 10/11] powerpc: Protect patching

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Date:   Fri, 07 May 2021 15:03:51 -0500
From:   "Christopher M. Riedl" <cmr@...ux.ibm.com>
To:     "Peter Zijlstra" <peterz@...radead.org>
Cc:     <tglx@...utronix.de>, <x86@...nel.org>,
        <linuxppc-dev@...ts.ozlabs.org>, <linux-hardening@...r.kernel.org>,
        <keescook@...omium.org>
Subject: Re: [RESEND PATCH v4 10/11] powerpc: Protect patching_mm with a
 lock

On Thu May 6, 2021 at 5:51 AM CDT, Peter Zijlstra wrote:
> On Wed, May 05, 2021 at 11:34:51PM -0500, Christopher M. Riedl wrote:
> > Powerpc allows for multiple CPUs to patch concurrently. When patching
> > with STRICT_KERNEL_RWX a single patching_mm is allocated for use by all
> > CPUs for the few times that patching occurs. Use a spinlock to protect
> > the patching_mm from concurrent use.
> > 
> > Modify patch_instruction() to acquire the lock, perform the patch op,
> > and then release the lock.
> > 
> > Also introduce {lock,unlock}_patching() along with
> > patch_instruction_unlocked() to avoid per-iteration lock overhead when
> > patch_instruction() is called in a loop. A follow-up patch converts some
> > uses of patch_instruction() to use patch_instruction_unlocked() instead.
>
> x86 uses text_mutex for all this, why not do the same?

I wasn't entirely sure if there is a problem with potentially going to
sleep in some of the places where patch_instruction() is called - the
spinlock avoids that (hypothetical) problem.

I just tried switching to text_mutex and at least on a P9 machine the
series boots w/ the Hash and Radix MMUs (with some lockdep errors). I
can rework this in the next version to use text_mutex if I don't find
any new problems with more extensive testing. It does mean more changes
to use patch_instruction_unlocked() in kprobe/optprobe/ftace in
arch/powerpc since iirc those are called with text_mutex already held.

Thanks!
Chris R.