Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <b3464e21-95e8-a80e-bf98-3c90c06afb91@intel.com>
Date:   Fri, 5 Aug 2022 10:24:04 -0700
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.11.0
Subject: Re: [PATCH v2] x86/fpu: Allow PKRU to be (once again) written by
 ptrace.
Content-Language: en-US
To:     Kyle Huey <me@kylehuey.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>
Cc:     Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org,
        Robert O'Callahan <robert@ocallahan.org>,
        David Manouchehri <david.manouchehri@riseup.net>,
        Borislav Petkov <bp@suse.de>, kvm@vger.kernel.org,
        stable@vger.kernel.org
References: <20220804031632.52921-1-khuey@kylehuey.com>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <20220804031632.52921-1-khuey@kylehuey.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 8/3/22 20:16, Kyle Huey wrote:
> When management of the PKRU register was moved away from XSTATE, emulation
> of PKRU's existence in XSTATE was added for APIs that read XSTATE, but not
> for APIs that write XSTATE. This can be seen by running gdb and executing
> `p $pkru`, `set $pkru = 42`, and `p $pkru`. On affected kernels (5.14+) the
> write to the PKRU register (which gdb performs through ptrace) is ignored.

Do you happen to have a reproducer for this sitting around?  I'd love to
get an addition to the pkeys selftest/ in place to make sure we don't
break this again.  PKRU is a very special snowflake.