Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Received-SPF: None (esa6.hc3370-68.iphmx.com: no sender
  authenticity information available from domain of
  andrew.cooper3@citrix.com) identity=pra;
  client-ip=162.221.158.21; receiver=esa6.hc3370-68.iphmx.com;
  envelope-from="Andrew.Cooper3@citrix.com";
  x-sender="andrew.cooper3@citrix.com";
  x-conformance=sidf_compatible
Received-SPF: Pass (esa6.hc3370-68.iphmx.com: domain of
  Andrew.Cooper3@citrix.com designates 162.221.158.21 as
  permitted sender) identity=mailfrom;
  client-ip=162.221.158.21; receiver=esa6.hc3370-68.iphmx.com;
  envelope-from="Andrew.Cooper3@citrix.com";
  x-sender="Andrew.Cooper3@citrix.com";
  x-conformance=sidf_compatible; x-record-type="v=spf1";
  x-record-text="v=spf1 ip4:209.167.231.154 ip4:178.63.86.133
  ip4:195.66.111.40/30 ip4:85.115.9.32/28 ip4:199.102.83.4
  ip4:192.28.146.160 ip4:192.28.146.107 ip4:216.52.6.88
  ip4:216.52.6.188 ip4:162.221.158.21 ip4:162.221.156.83
  ip4:168.245.78.127 ~all"
Received-SPF: None (esa6.hc3370-68.iphmx.com: no sender
  authenticity information available from domain of
  postmaster@mail.citrix.com) identity=helo;
  client-ip=162.221.158.21; receiver=esa6.hc3370-68.iphmx.com;
  envelope-from="Andrew.Cooper3@citrix.com";
  x-sender="postmaster@mail.citrix.com";
  x-conformance=sidf_compatible
IronPort-SDR: 2WpCl0BcYn7VtPh8E/1pwTYpPhQ+Ve3kSiEGQX66QXjIUenNN7fQhb1tzp2OY+jzrGgSABcxVS
 yqZB39Ae0uBLWqbyL4I1sEb+jgEvIdvWqTtz/PdqK18zTzNQwwHsKAdtCz5LukEx9gz8lpwxx2
 qfG/cTKIZvGACNIJBRWzCD893s+ihHnbOQtiuVUyttMcuT248nt/o26o3sHwq6XG0+QBVydFKL
 vEV7utaCAm2gMvVLJ9eGh0Mw3vFEyMKtwzXJMfRCwBbJxabY/zQAMzlJ9uqJVemIfpfIzWZWR6
 Png=
Subject: Re: [Xen-devel] [PATCH] x86/stackframe/32: repair 32-bit Xen PV
To:     Andy Lutomirski <luto@kernel.org>, Jan Beulich <jbeulich@suse.com>,
        xen-devel <xen-devel@lists.xenproject.org>,
        X86 ML <x86@kernel.org>,
        "Peter Zijlstra" <peterz@infradead.org>
CC:     lkml <linux-kernel@vger.kernel.org>
References: <ef1c9381-dfc7-7150-feca-581f4d798513@suse.com>
 <CALCETrWAALF7EgxHGs-rtZwk1Fxttr56QKXeB6QssXbyXDs+kA@mail.gmail.com>
From:   Andrew Cooper <andrew.cooper3@citrix.com>
Openpgp: preference=signencrypt
Autocrypt: addr=andrew.cooper3@citrix.com; prefer-encrypt=mutual; keydata=
 mQINBFLhNn8BEADVhE+Hb8i0GV6mihnnr/uiQQdPF8kUoFzCOPXkf7jQ5sLYeJa0cQi6Penp
 VtiFYznTairnVsN5J+ujSTIb+OlMSJUWV4opS7WVNnxHbFTPYZVQ3erv7NKc2iVizCRZ2Kxn
 srM1oPXWRic8BIAdYOKOloF2300SL/bIpeD+x7h3w9B/qez7nOin5NzkxgFoaUeIal12pXSR
 Q354FKFoy6Vh96gc4VRqte3jw8mPuJQpfws+Pb+swvSf/i1q1+1I4jsRQQh2m6OTADHIqg2E
 ofTYAEh7R5HfPx0EXoEDMdRjOeKn8+vvkAwhviWXTHlG3R1QkbE5M/oywnZ83udJmi+lxjJ5
 YhQ5IzomvJ16H0Bq+TLyVLO/VRksp1VR9HxCzItLNCS8PdpYYz5TC204ViycobYU65WMpzWe
 LFAGn8jSS25XIpqv0Y9k87dLbctKKA14Ifw2kq5OIVu2FuX+3i446JOa2vpCI9GcjCzi3oHV
 e00bzYiHMIl0FICrNJU0Kjho8pdo0m2uxkn6SYEpogAy9pnatUlO+erL4LqFUO7GXSdBRbw5
 gNt25XTLdSFuZtMxkY3tq8MFss5QnjhehCVPEpE6y9ZjI4XB8ad1G4oBHVGK5LMsvg22PfMJ
 ISWFSHoF/B5+lHkCKWkFxZ0gZn33ju5n6/FOdEx4B8cMJt+cWwARAQABtClBbmRyZXcgQ29v
 cGVyIDxhbmRyZXcuY29vcGVyM0BjaXRyaXguY29tPokCOgQTAQgAJAIbAwULCQgHAwUVCgkI
 CwUWAgMBAAIeAQIXgAUCWKD95wIZAQAKCRBlw/kGpdefoHbdD/9AIoR3k6fKl+RFiFpyAhvO
 59ttDFI7nIAnlYngev2XUR3acFElJATHSDO0ju+hqWqAb8kVijXLops0gOfqt3VPZq9cuHlh
 IMDquatGLzAadfFx2eQYIYT+FYuMoPZy/aTUazmJIDVxP7L383grjIkn+7tAv+qeDfE+txL4
 SAm1UHNvmdfgL2/lcmL3xRh7sub3nJilM93RWX1Pe5LBSDXO45uzCGEdst6uSlzYR/MEr+5Z
 JQQ32JV64zwvf/aKaagSQSQMYNX9JFgfZ3TKWC1KJQbX5ssoX/5hNLqxMcZV3TN7kU8I3kjK
 mPec9+1nECOjjJSO/h4P0sBZyIUGfguwzhEeGf4sMCuSEM4xjCnwiBwftR17sr0spYcOpqET
 ZGcAmyYcNjy6CYadNCnfR40vhhWuCfNCBzWnUW0lFoo12wb0YnzoOLjvfD6OL3JjIUJNOmJy
 RCsJ5IA/Iz33RhSVRmROu+TztwuThClw63g7+hoyewv7BemKyuU6FTVhjjW+XUWmS/FzknSi
 dAG+insr0746cTPpSkGl3KAXeWDGJzve7/SBBfyznWCMGaf8E2P1oOdIZRxHgWj0zNr1+ooF
 /PzgLPiCI4OMUttTlEKChgbUTQ+5o0P080JojqfXwbPAyumbaYcQNiH1/xYbJdOFSiBv9rpt
 TQTBLzDKXok86LkCDQRS4TZ/ARAAkgqudHsp+hd82UVkvgnlqZjzz2vyrYfz7bkPtXaGb9H4
 Rfo7mQsEQavEBdWWjbga6eMnDqtu+FC+qeTGYebToxEyp2lKDSoAsvt8w82tIlP/EbmRbDVn
 7bhjBlfRcFjVYw8uVDPptT0TV47vpoCVkTwcyb6OltJrvg/QzV9f07DJswuda1JH3/qvYu0p
 vjPnYvCq4NsqY2XSdAJ02HrdYPFtNyPEntu1n1KK+gJrstjtw7KsZ4ygXYrsm/oCBiVW/OgU
 g/XIlGErkrxe4vQvJyVwg6YH653YTX5hLLUEL1NS4TCo47RP+wi6y+TnuAL36UtK/uFyEuPy
 wwrDVcC4cIFhYSfsO0BumEI65yu7a8aHbGfq2lW251UcoU48Z27ZUUZd2Dr6O/n8poQHbaTd
 6bJJSjzGGHZVbRP9UQ3lkmkmc0+XCHmj5WhwNNYjgbbmML7y0fsJT5RgvefAIFfHBg7fTY/i
 kBEimoUsTEQz+N4hbKwo1hULfVxDJStE4sbPhjbsPCrlXf6W9CxSyQ0qmZ2bXsLQYRj2xqd1
 bpA+1o1j2N4/au1R/uSiUFjewJdT/LX1EklKDcQwpk06Af/N7VZtSfEJeRV04unbsKVXWZAk
 uAJyDDKN99ziC0Wz5kcPyVD1HNf8bgaqGDzrv3TfYjwqayRFcMf7xJaL9xXedMcAEQEAAYkC
 HwQYAQgACQUCUuE2fwIbDAAKCRBlw/kGpdefoG4XEACD1Qf/er8EA7g23HMxYWd3FXHThrVQ
 HgiGdk5Yh632vjOm9L4sd/GCEACVQKjsu98e8o3ysitFlznEns5EAAXEbITrgKWXDDUWGYxd
 pnjj2u+GkVdsOAGk0kxczX6s+VRBhpbBI2PWnOsRJgU2n10PZ3mZD4Xu9kU2IXYmuW+e5KCA
 vTArRUdCrAtIa1k01sPipPPw6dfxx2e5asy21YOytzxuWFfJTGnVxZZSCyLUO83sh6OZhJkk
 b9rxL9wPmpN/t2IPaEKoAc0FTQZS36wAMOXkBh24PQ9gaLJvfPKpNzGD8XWR5HHF0NLIJhgg
 4ZlEXQ2fVp3XrtocHqhu4UZR4koCijgB8sB7Tb0GCpwK+C4UePdFLfhKyRdSXuvY3AHJd4CP
 4JzW0Bzq/WXY3XMOzUTYApGQpnUpdOmuQSfpV9MQO+/jo7r6yPbxT7CwRS5dcQPzUiuHLK9i
 nvjREdh84qycnx0/6dDroYhp0DFv4udxuAvt1h4wGwTPRQZerSm4xaYegEFusyhbZrI0U9tJ
 B8WrhBLXDiYlyJT6zOV2yZFuW47VrLsjYnHwn27hmxTC/7tvG3euCklmkn9Sl9IAKFu29RSo
 d5bD8kMSCYsTqtTfT6W4A3qHGvIDta3ptLYpIAOD2sY3GYq2nf3Bbzx81wZK14JdDDHUX2Rs
 6+ahAA==
Message-ID: <8f9f812b-c28a-5828-d8d9-37ae7e2f99da@citrix.com>
Date:   Thu, 24 Oct 2019 17:31:57 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.9.0
MIME-Version: 1.0
In-Reply-To: <CALCETrWAALF7EgxHGs-rtZwk1Fxttr56QKXeB6QssXbyXDs+kA@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 8bit
Content-Language: en-GB
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 24/10/2019 17:11, Andy Lutomirski wrote:
>> +# define USER_SEGMENT_RPL_MASK (SEGMENT_RPL_MASK & ~1)
>> +#endif
>> +
>>         .section .entry.text, "ax"
>>
>>  /*
>> @@ -172,7 +183,7 @@
>>         ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI
>>         .if \no_user_check == 0
>>         /* coming from usermode? */
>> -       testl   $SEGMENT_RPL_MASK, PT_CS(%esp)
>> +       testl   $USER_SEGMENT_RPL_MASK, PT_CS(%esp)
> Shouldn't PT_CS(%esp) be 0 if we came from the kernel?  I'm guessing
> the actual bug is in whatever code put 1 in here in the first place.

Ring1 kernels (32bit) consistently see RPL1 everywhere under Xen.

Back in the days of a 32bit Xen, int $0x80 really was wired directly
from ring 3 to 1, and didn't bounce through Xen.  This isn't possible in
long mode, because all IDT gates are required to be 64bit code segments.

Ring3 kernels (64bit) consistently see RPL0 everywhere under Xen,
because presumably this was less invasive when designing the ABI.

~Andrew