From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.zytor.com (terminus.zytor.com [198.137.202.136]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D5C271CA8D for ; Sun, 15 Sep 2024 18:18:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.137.202.136 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1726424314; cv=none; b=V5m4cmn/K6F0Prk6k1PqhLLL0xkK8wEckgW5N3Kq8oaY9i2IgWcND9cZZ9j3cMjBtWsB4+U3f2TLdWwrHSQqARoWTRT+KokGNEQWmbwf37NO4R2t0hXHc9aUbj4rNWE2j0bvRuavqT0gnQwR8jP+FeU7ai0/YIeKXZ0pFdtFAdc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1726424314; c=relaxed/simple; bh=75/r36ucxqPxddXPx/LTiELdA2WDjbIUiMuuV5FEDQo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=tQbitvu2Id8JvUQxycYXFz7q0ZFXUUTJWBK3awBgp25Vcd7I79GdSu/YtsmBvjx534fp0lv8cg6qvCNHb4Uf/OY9rw6fXdT0q4/k3CaRI31JilGOYRYDrEojvEfvDQ3ACCjSPIsD3LRc7Q3xAQUc/1drYIOXoElrm0yI8EqBryw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=zytor.com; spf=pass smtp.mailfrom=zytor.com; dkim=pass (2048-bit key) header.d=zytor.com header.i=@zytor.com header.b=I3yofWwo; arc=none smtp.client-ip=198.137.202.136 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=zytor.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=zytor.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=zytor.com header.i=@zytor.com header.b="I3yofWwo" Received: from [192.168.7.205] ([71.202.166.45]) (authenticated bits=0) by mail.zytor.com (8.18.1/8.17.1) with ESMTPSA id 48FIHQQR4192878 (version=TLSv1.3 cipher=TLS_AES_128_GCM_SHA256 bits=128 verify=NO); Sun, 15 Sep 2024 11:17:27 -0700 DKIM-Filter: OpenDKIM Filter v2.11.0 mail.zytor.com 48FIHQQR4192878 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zytor.com; s=2024081601; t=1726424248; bh=EPPramTpNOw21EZs2IRyI4Db2QwHDqx+ygb5iuhq/3w=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=I3yofWwohFr0b6LvzyjOcQl9VYw/XWVLYHx+oTXQqw+1d0gqDGvsDIpLIvTRTZkCh FBv5d7ND8bfjmBedvBMRg9PqB/kbnMFoG10np3t+x3VB+7sxPh5iiZDTpbuF30eICb SNsJzUsUSXgzBiANy948iptj5dp81YWWbfaF9cVbo6Vwx5PD/aPnhJ1+mehbpulWeJ BaCkke5UD2msee9QV9vcH9PP/Bkh6wUC2hqGWiqWoniJxz5UOfbrL3jGOEntZxxfGt EAROpVOZvmQG0uDQ7AJgfCETjwCBAsyRUr+3jODI7CpuKG9rvkMDnzzo+V6HRf976Q Zq+BpGm4Hhk1A== Message-ID: <5cc939d6-c5f5-410c-b7a2-3e96fe7f1fae@zytor.com> Date: Sun, 15 Sep 2024 11:17:26 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 1/1] x86/fred: Clear the WFE bit in missing-ENDBRANCH #CP To: Andrew Cooper , linux-kernel@vger.kernel.org Cc: tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, peterz@infradead.org References: <20240911231929.2632698-1-xin@zytor.com> <04674df0-a026-42d1-aacf-293f3018329f@citrix.com> Content-Language: en-US From: Xin Li Autocrypt: addr=xin@zytor.com; keydata= xsDNBGUPz1cBDACS/9yOJGojBFPxFt0OfTWuMl0uSgpwk37uRrFPTTLw4BaxhlFL0bjs6q+0 2OfG34R+a0ZCuj5c9vggUMoOLdDyA7yPVAJU0OX6lqpg6z/kyQg3t4jvajG6aCgwSDx5Kzg5 Rj3AXl8k2wb0jdqRB4RvaOPFiHNGgXCs5Pkux/qr0laeFIpzMKMootGa4kfURgPhRzUaM1vy bsMsL8vpJtGUmitrSqe5dVNBH00whLtPFM7IbzKURPUOkRRiusFAsw0a1ztCgoFczq6VfAVu raTye0L/VXwZd+aGi401V2tLsAHxxckRi9p3mc0jExPc60joK+aZPy6amwSCy5kAJ/AboYtY VmKIGKx1yx8POy6m+1lZ8C0q9b8eJ8kWPAR78PgT37FQWKYS1uAroG2wLdK7FiIEpPhCD+zH wlslo2ETbdKjrLIPNehQCOWrT32k8vFNEMLP5G/mmjfNj5sEf3IOKgMTMVl9AFjsINLHcxEQ 6T8nGbX/n3msP6A36FDfdSEAEQEAAc0WWGluIExpIDx4aW5Aenl0b3IuY29tPsLBDQQTAQgA NxYhBIUq/WFSDTiOvUIqv2u9DlcdrjdRBQJlD89XBQkFo5qAAhsDBAsJCAcFFQgJCgsFFgID AQAACgkQa70OVx2uN1HUpgv/cM2fsFCQodLArMTX5nt9yqAWgA5t1srri6EgS8W3F+3Kitge tYTBKu6j5BXuXaX3vyfCm+zajDJN77JHuYnpcKKr13VcZi1Swv6Jx1u0II8DOmoDYLb1Q2ZW v83W55fOWJ2g72x/UjVJBQ0sVjAngazU3ckc0TeNQlkcpSVGa/qBIHLfZraWtdrNAQT4A1fa sWGuJrChBFhtKbYXbUCu9AoYmmbQnsx2EWoJy3h7OjtfFapJbPZql+no5AJ3Mk9eE5oWyLH+ QWqtOeJM7kKvn/dBudokFSNhDUw06e7EoVPSJyUIMbYtUO7g2+Atu44G/EPP0yV0J4lRO6EA wYRXff7+I1jIWEHpj5EFVYO6SmBg7zF2illHEW31JAPtdDLDHYcZDfS41caEKOQIPsdzQkaQ oW2hchcjcMPAfyhhRzUpVHLPxLCetP8vrVhTvnaZUo0xaVYb3+wjP+D5j/3+hwblu2agPsaE vgVbZ8Fx3TUxUPCAdr/p73DGg57oHjgezsDNBGUPz1gBDAD4Mg7hMFRQqlzotcNSxatlAQNL MadLfUTFz8wUUa21LPLrHBkUwm8RujehJrzcVbPYwPXIO0uyL/F///CogMNx7Iwo6by43KOy g89wVFhyy237EY76j1lVfLzcMYmjBoTH95fJC/lVb5Whxil6KjSN/R/y3jfG1dPXfwAuZ/4N cMoOslWkfZKJeEut5aZTRepKKF54T5r49H9F7OFLyxrC/uI9UDttWqMxcWyCkHh0v1Di8176 jjYRNTrGEfYfGxSp+3jYL3PoNceIMkqM9haXjjGl0W1B4BidK1LVYBNov0rTEzyr0a1riUrp Qk+6z/LHxCM9lFFXnqH7KWeToTOPQebD2B/Ah5CZlft41i8L6LOF/LCuDBuYlu/fI2nuCc8d m4wwtkou1Y/kIwbEsE/6RQwRXUZhzO6llfoN96Fczr/RwvPIK5SVMixqWq4QGFAyK0m/1ap4 bhIRrdCLVQcgU4glo17vqfEaRcTW5SgX+pGs4KIPPBE5J/ABD6pBnUUAEQEAAcLA/AQYAQgA JhYhBIUq/WFSDTiOvUIqv2u9DlcdrjdRBQJlD89ZBQkFo5qAAhsMAAoJEGu9DlcdrjdR4C0L /RcjolEjoZW8VsyxWtXazQPnaRvzZ4vhmGOsCPr2BPtMlSwDzTlri8BBG1/3t/DNK4JLuwEj OAIE3fkkm+UG4Kjud6aNeraDI52DRVCSx6xff3bjmJsJJMb12mWglN6LjdF6K+PE+OTJUh2F dOhslN5C2kgl0dvUuevwMgQF3IljLmi/6APKYJHjkJpu1E6luZec/lRbetHuNFtbh3xgFIJx 2RpgVDP4xB3f8r0I+y6ua+p7fgOjDLyoFjubRGed0Be45JJQEn7A3CSb6Xu7NYobnxfkwAGZ Q81a2XtvNS7Aj6NWVoOQB5KbM4yosO5+Me1V1SkX2jlnn26JPEvbV3KRFcwV5RnDxm4OQTSk PYbAkjBbm+tuJ/Sm+5Yp5T/BnKz21FoCS8uvTiziHj2H7Cuekn6F8EYhegONm+RVg3vikOpn gao85i4HwQTK9/D1wgJIQkdwWXVMZ6q/OALaBp82vQ2U9sjTyFXgDjglgh00VRAHP7u1Rcu4 l75w1xInsg== In-Reply-To: <04674df0-a026-42d1-aacf-293f3018329f@citrix.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 9/11/2024 4:35 PM, Andrew Cooper wrote: > On 12/09/2024 12:19 am, Xin Li (Intel) wrote: >> The WFE, i.e., WAIT_FOR_ENDBRANCH, bit in the augmented CS of FRED >> stack frame is set to 1 in missing-ENDBRANCH #CP exceptions. >> >> The CPU will generate another missing-ENDBRANCH #CP if the WFE bit >> is left as 1, because the indirect branch tracker will be set in >> the WAIT_FOR_ENDBRANCH state upon completion of the following ERETS >> instruction and the CPU will restart from the IP that just caused a >> previous missing-ENDBRANCH #CP. >> >> Clear the WFE bit to avoid dead looping in missing-ENDBRANCH #CP. >> >> Signed-off-by: Xin Li (Intel) > > Ah - good.  Finally some evidence that this hole in CET has been plugged > by FRED. > > However, I'd suggest describing it differently. > > > By definition, all missing-ENDBRANCH #CPs are a result of WFE && !ENDBR. > > But, in original CET under IDT delivery, any transfer for > interrupt/exception/etc that does not change privilege will clobber the > WFE state because MSR_{U,S}_CET.WFE is intentionally set by microcode so > as to expect to find an ENDBR at the interrupt/exception/syscall entrypoint. > > In practice, this means interrupts and exceptions hitting the kernel, or > user interrupts, loose the WFE state of the interrupted context.  And loose -> lose? Xin > yes, this means that a well timed interrupt (to the precise instruction > boundary) will let an attacker sneak a bad function pointer past the > CET-IBT enforcement. > > In FRED, the WFE state of the interrupted context (even if it is the > same privilege) is preserved and restored, in order to close this hole. > > Therefore, the intentional #CP selfchecks need to clear WFE when they > are deemed to have succeeded, now that FRED is causing the state not to > get lost. > > ~Andrew