From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=170.10.129.124 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1792789252; b=HQfKIVeD51gAxCp0BMMHOwhZXsqMPzeug1119CQmzgYH/BVXbiPm1j42HYIV+pkvgwwH pwecMK+t/Uuk1Sw8XsmRhhu8uyLux/s1je33DmHR5gFG2zo6qgVJ7URljMl5ZB02RKYnl qdKcQ6ul58s2OKyJwi9cAdKvmY557wb4XgrokMsNhS8BO8D2tx0FkQJ0He2+iu9F9LjTT 48WhHtQQDdkrg8SOecpwBy0mmBgz7rHmryxHdzNxRsGdA4JYV+Y017A3fNTpn3HjoHaYk /7KzJa+d2qEgJTWqRyFcJ9d4qPd9pHKnc427ulHInjk6QccvA7iOtzQJrGRL27fy7Qg== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1792789252; h=DMARC-Filter:DKIM-Signature:Received:X-MC-Unique: X-Mimecast-MFC-AGG-ID:Received:Received:Message-ID:Date:MIME-Version: User-Agent:Subject:To:References:From:Content-Language:Autocrypt: In-Reply-To:Content-Type:Content-Transfer-Encoding:X-Scanned-By; bh=fO9tCv+Dk6N+Wu3Qpc9SroVvJnnr+VUFnmZ0sB7IXg8=; b=DRJfsnh6jcMfKA/g4mD+WM2oqDBvvagLccLc/DMY9bB1jfAh/i5FTT598qr0hehfgMuF 3ZPbY1PxoFWiMshQ077A6f6N0TBGdOVCezdF/uSbiIzvkKGsNb+53YSxCpWHtLuhI8i0Q iZNIpcQTUoBj3fI43eKcEGyR0n96VJk0PRaRYtxV+9s9besiqU8tf455541ez6/8e8p0W qWW/debhAy8+mKBMZEaJvZ4+o62hD47LaFhExLGR5BO6exUy9pzIRf7EZJ8EL4oMstCVd TLlshRwv+woYKwDoaILttDBMQTcgo/fwNNiHXWmsFNO1VCxFyOgEKwDezYfs1w1drDQ== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256 header.s=mimecast20190719 header.b="bQBAb/l0"; arc=none smtp.remote-ip=170.10.129.124 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 78DC2606A31A for ; Fri, 23 Oct 2026 21:00:51 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 78DC2606A31A Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256 header.s=mimecast20190719 header.b=bQBAb/l0 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1792789250; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:autocrypt:autocrypt; bh=fO9tCv+Dk6N+Wu3Qpc9SroVvJnnr+VUFnmZ0sB7IXg8=; b=bQBAb/l028iuTudHJmK/6mFP0/J0D1LSpkqtJV/k8jTTIKi+slUnkduMWvFyUP30KKIv0t PqWwfc372fK6kMYH92eN+T/6TRJa4SpiFqhhgjQ2/tMVZLGc6XmVMFz+45J7e9T5XGbb0+ FoaxfXrwqPxYOOlQ56tl5TeCpv6nbY0= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-99-lfVUmV-uPeW1doAq3MUDGA-1; Fri, 23 Oct 2026 17:00:43 -0400 X-MC-Unique: lfVUmV-uPeW1doAq3MUDGA-1 X-Mimecast-MFC-AGG-ID: lfVUmV-uPeW1doAq3MUDGA_1792789241 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id ACB7C18007F1; Fri, 23 Oct 2026 21:00:40 +0000 (UTC) Received: from [100.90.56.12] (headnet04.pony-001.prod.iad2.dc.redhat.com [10.2.32.116]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id D5D9F1800480; Fri, 23 Oct 2026 21:00:36 +0000 (UTC) Message-ID: Date: Fri, 23 Oct 2026 23:00:34 +0200 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Kernel Shadow Stacks Versus XFEATURE_MASK_GUEST_SUPERVISOR To: Dave Hansen , Yang Weijiang , "Gao, Chao" , "Edgecombe, Rick P" , Maxim Levitsky , John Allen , LKML , peterz@infradead.org, kvm@vger.kernel.org, binbin.wu@linux.intel.com, Andrew Cooper , Thomas Gleixner , "Bae, Chang Seok" , "Mehta, Sohil" References: <2f0560df-69fc-4fc1-81fa-447e289ec2df@intel.com> From: Paolo Bonzini Content-Language: en-US Autocrypt: addr=pbonzini@redhat.com; keydata= xsEhBFRCcBIBDqDGsz4K0zZun3jh+U6Z9wNGLKQ0kSFyjN38gMqU1SfP+TUNQepFHb/Gc0E2 CxXPkIBTvYY+ZPkoTh5xF9oS1jqI8iRLzouzF8yXs3QjQIZ2SfuCxSVwlV65jotcjD2FTN04 hVopm9llFijNZpVIOGUTqzM4U55sdsCcZUluWM6x4HSOdw5F5Utxfp1wOjD/v92Lrax0hjiX DResHSt48q+8FrZzY+AUbkUS+Jm34qjswdrgsC5uxeVcLkBgWLmov2kMaMROT0YmFY6A3m1S P/kXmHDXxhe23gKb3dgwxUTpENDBGcfEzrzilWueOeUWiOcWuFOed/C3SyijBx3Av/lbCsHU Vx6pMycNTdzU1BuAroB+Y3mNEuW56Yd44jlInzG2UOwt9XjjdKkJZ1g0P9dwptwLEgTEd3Fo UdhAQyRXGYO8oROiuh+RZ1lXp6AQ4ZjoyH8WLfTLf5g1EKCTc4C1sy1vQSdzIRu3rBIjAvnC tGZADei1IExLqB3uzXKzZ1BZ+Z8hnt2og9hb7H0y8diYfEk2w3R7wEr+Ehk5NQsT2MPI2QBd wEv1/Aj1DgUHZAHzG1QN9S8wNWQ6K9DqHZTBnI1hUlkp22zCSHK/6FwUCuYp1zcAEQEAAc0j UGFvbG8gQm9uemluaSA8cGJvbnppbmlAcmVkaGF0LmNvbT7CwU0EEwECACMFAlRCcBICGwMH CwkIBwMCAQYVCAIJCgsEFgIDAQIeAQIXgAAKCRB+FRAMzTZpsbceDp9IIN6BIA0Ol7MoB15E 11kRz/ewzryFY54tQlMnd4xxfH8MTQ/mm9I482YoSwPMdcWFAKnUX6Yo30tbLiNB8hzaHeRj jx12K+ptqYbg+cevgOtbLAlL9kNgLLcsGqC2829jBCUTVeMSZDrzS97ole/YEez2qFpPnTV0 VrRWClWVfYh+JfzpXmgyhbkuwUxNFk421s4Ajp3d8nPPFUGgBG5HOxzkAm7xb1cjAuJ+oi/K CHfkuN+fLZl/u3E/fw7vvOESApLU5o0icVXeakfSz0LsygEnekDbxPnE5af/9FEkXJD5EoYG SEahaEtgNrR4qsyxyAGYgZlS70vkSSYJ+iT2rrwEiDlo31MzRo6Ba2FfHBSJ7lcYdPT7bbk9 AO3hlNMhNdUhoQv7M5HsnqZ6unvSHOKmReNaS9egAGdRN0/GPDWr9wroyJ65ZNQsHl9nXBqE AukZNr5oJO5vxrYiAuuTSd6UI/xFkjtkzltG3mw5ao2bBpk/V/YuePrJsnPFHG7NhizrxttB nTuOSCMo45pfHQ+XYd5K1+Cv/NzZFNWscm5htJ0HznY+oOsZvHTyGz3v91pn51dkRYN0otqr bQ4tlFFuVjArBZcapSIe6NV8C4cEiSTOwE0EVEJx7gEIAMeHcVzuv2bp9HlWDp6+RkZe+vtl KwAHplb/WH59j2wyG8V6i33+6MlSSJMOFnYUCCL77bucx9uImI5nX24PIlqT+zasVEEVGSRF m8dgkcJDB7Tps0IkNrUi4yof3B3shR+vMY3i3Ip0e41zKx0CvlAhMOo6otaHmcxr35sWq1Jk tLkbn3wG+fPQCVudJJECvVQ//UAthSSEklA50QtD2sBkmQ14ZryEyTHQ+E42K3j2IUmOLriF dNr9NvE1QGmGyIcbw2NIVEBOK/GWxkS5+dmxM2iD4Jdaf2nSn3jlHjEXoPwpMs0KZsgdU0pP JQzMUMwmB1wM8JxovFlPYrhNT9MAEQEAAcLBMwQYAQIACQUCVEJx7gIbDAAKCRB+FRAMzTZp sadRDqCctLmYICZu4GSnie4lKXl+HqlLanpVMOoFNnWs9oRP47MbE2wv8OaYh5pNR9VVgyhD OG0AU7oidG36OeUlrFDTfnPYYSF/mPCxHttosyt8O5kabxnIPv2URuAxDByz+iVbL+RjKaGM GDph56ZTswlx75nZVtIukqzLAQ5fa8OALSGum0cFi4ptZUOhDNz1onz61klD6z3MODi0sBZN Aj6guB2L/+2ZwElZEeRBERRd/uommlYuToAXfNRdUwrwl9gRMiA0WSyTb190zneRRDfpSK5d usXnM/O+kr3Dm+Ui+UioPf6wgbn3T0o6I5BhVhs4h4hWmIW7iNhPjX1iybXfmb1gAFfjtHfL xRUr64svXpyfJMScIQtBAm0ihWPltXkyITA92ngCmPdHa6M1hMh4RDX+Jf1fiWubzp1voAg0 JBrdmNZSQDz0iKmSrx8xkoXYfA3bgtFN8WJH2xgFL28XnqY4M6dLhJwV3z08tPSRqYFm4NMP dRsn0/7oymhneL8RthIvjDDQ5ktUjMe8LtHr70OZE/TT88qvEdhiIVUogHdo4qBrk41+gGQh b906Dudw5YhTJFU3nC6bbF2nrLlB4C/XSiH76ZvqzV0Z/cAMBo5NF/w= In-Reply-To: <2f0560df-69fc-4fc1-81fa-447e289ec2df@intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 On 10/23/26 21:03, Dave Hansen wrote: > So, I'm working on kernel shadow stacks. I have some nice optimization > code like fred_update_rsp0() for SSP0. It avoids WRMSR's on exits to > userspace if FRED_SSP0 hasn't changed, just like FRED_RSP0. > > Well, turns out that FRED_SSP0 isn't like FRED_RSP0 in one very > important way: it is an alias for PL0_SSP, which is managed via > XFEATURE_CET_KERNEL which is in: > > /* Supervisor features which are enabled only in guest FPUs */ > #define XFEATURE_MASK_GUEST_SUPERVISOR XFEATURE_MASK_CET_KERNEL > > So, I was running along just fine in my nice shadow stack kernel going > in and out of the kernel just fine. But the first time I did an XRSTOR, > the XFEATURE_CET_KERNEL feature got set to its init state. My SSP0 cache > didn't notice and I tried to run userspace with SSP0=0. Much hilarity > ensued on the next kernel entry. > > The most adorable part of all this is that I have KVM compiled > completely out on my config. The "only in guest FPUs" code is biting me > on a kernel that can't even run guests. If you want to quickly work around for your !CONFIG_KVM kernel, I would do #ifdef CONFIG_KVM /* Supervisor features which are enabled only in guest FPUs */ #define XFEATURE_MASK_GUEST_SUPERVISOR XFEATURE_MASK_CET_KERNEL #else #define XFEATURE_MASK_GUEST_SUPERVISOR 0 #endif and a small adjustment like BUILD_BUG_ON((XFEATURE_MASK_USER_SUPPORTED | XFEATURE_MASK_SUPERVISOR_SUPPORTED) & ~XFEATURES_INIT_FPSTATE_HANDLED); That will get rid of the bit in XSS. > Any thoughts on what we should do? > > Actually context switching XFEATURE_CET_KERNEL seems kinda silly in a > lot of ways. PL0_SSP can always be derived _and_ it isn't actually even > needed at context switch time, only when running userspace. True but, just to understand, what is the performance like if you actually use XSAVES/XRSTORS to update SSP0 on context switches instead of WRMSR+cache? Hopefully the microcode does a bunch of optimizations as well. > I could just aggressively blow away my SSP0 cache any time an XRSTOR > touches XFEATURE_CET_KERNEL. That would be easy. But it also seems silly > that XFEATURE_CET_KERNEL is being blown away at _all_ during normal > context switches. Heck, it goes further than that because: > > fpregs_restore_userregs() does: > > restore_fpregs_from_fpstate(fpu->fpstate, > XFEATURE_MASK_FPSTATE); > > Where XFEATURE_MASK_FPSTATE contains XFEATURE_MASK_SUPERVISOR_SUPPORTED. The actual mask used by restore_fpregs_from_fpstate is tighter though, it's "fpu_kernel_cfg.max_features & XFEATURE_MASK_FPSTATE". Can you replace fpu_kernel_cfg.max_features with some fpstate->xyz (sorry too late to think it through :)) that has CET_KERNEL for the guest FPU and not for the regular one? > So a "restore_userregs" function is blowing away supervisor state. > > IOW, I think we're using XFEATURE_MASK_FPSTATE a bit too broadly. > > I'd like to get to a place where only the KVM code even has > XFEATURE_CET_KERNEL in its RFBM. That I agree, but I think it can be handled in {save,restore}_fpregs_to_fpstate(). Paolo