linux-kernel - Re: Memory protection keys: Signal handlers crash if pkey0 is write-disabled

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Message-ID: <c0159d08-e69d-0329-5ca9-68fd26cab0c8@intel.com>
Date:   Thu, 7 Sep 2023 14:31:13 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Robert Kueffner <r.m.kueffner@...il.com>,
        Kyle Huey <me@...ehuey.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: Memory protection keys: Signal handlers crash if pkey0 is
 write-disabled

On 9/7/23 14:22, Robert Kueffner wrote:
> Is there some way to make this work, or is it generally not possible
> to successfully handle exceptions if WD0=true?

It's theoretically possible, but it's in a grey area.  The kernel can't
easily try to respect PKRU *and* override it for things like decoding
userspace instructions.

PKRU should get reset to a value that permits reads and writes to pkey-0
before the signal frame is created.  But you're obviously tripping over
it anyway.

I assume that *something* is trying to access pkey-0-protected memory.
Any idea what that is?  Which entity is doing that access and what are
they accessing?  The page fault tracepoints might come in handy.