linux-kernel - Re: [RFC PATCH] x86/pkeys: update PKRU to enable pkey 0 before XSAVE

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Date: Thu, 14 Mar 2024 18:14:49 +0000
From: Aruna Ramakrishna <aruna.ramakrishna@...cle.com>
To: Dave Hansen <dave.hansen@...el.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: [RFC PATCH] x86/pkeys: update PKRU to enable pkey 0 before XSAVE

> On Mar 14, 2024, at 10:54 AM, Dave Hansen <dave.hansen@...el.com> wrote:
> 
> On 3/14/24 10:29, Aruna Ramakrishna wrote:
>> This patch is a workaround for a bug where the PKRU value is not
>> restored to the init value before the signal handler is invoked.
> 
> I don't think we should touch this with a ten foot pole without a test
> for it in tools/testing/selftests/mm/protection_keys.c.

I’ll add a test case here.

> 
> I'm not sure this is worth the trouble.  Protection keys is not a
> security feature.  This isn't a regression.  It's been the behavior
> since day one.  This really is a feature request for a behavioral
> improvement, not a bug fix.
> 
> The need for this new feature is highly dependent on the threat model
> that it supports.  I'm highly dubious that there's a true need to
> protect against an attacker with arbitrary write access in the same
> address space.  We need to have a lot more information there.

I thought the PKRU value being reset in the signal handler was supposed to be the default behavior. In which case, this is a bug.

"Signal Handler Behavior
Each time a signal handler is invoked (including nested signals),
the thread is temporarily given a new, default set of protection
key rights that override the rights from the interrupted context.”

(Ref: https://man7.org/linux/man-pages/man7/pkeys.7.html)

I'm not very familiar with protection keys (before I started looking into this issue), so I apologize for misunderstanding.

fpu__clear_user_states() does reset PKRU, but that happens much later in the flow. Before that, the kernel tries to save registers on to the alternate signal stack in setup_rt_frame(), and that fails if the application has explicitly disabled pkey 0 (and the alt stack is protected by pkey 0). This patch attempts to move that reset a little earlier in the flow, so that setup_rt_frame() can succeed.

> 
> I haven't even more than glanced at the code.  It looks pretty
> unspeakably ugly even at a glance.

I agree with you - no argument there.

But I’m not sure there is a “clean” way to do this. If there is, I’m happy to redo the patch.

Thanks,
Aruna