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X-CSE-ConnectionGUID: zMHEF3PdSCql9XT51gcwxA== X-CSE-MsgGUID: nFNGA4DvT0umg5MJY2RHAQ== X-IronPort-AV: E=McAfee;i="6700,10204,11222"; a="53971398" X-IronPort-AV: E=Sophos;i="6.11,199,1725346800"; d="scan'208";a="53971398" Received: from orviesa002.jf.intel.com ([10.64.159.142]) by orvoesa101.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 13 Nov 2024 09:49:01 -0800 X-CSE-ConnectionGUID: 0QFIAp/FRUaORCiagoFLOA== X-CSE-MsgGUID: 9v9RNzvxRnW8iv/lPwjPsg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.12,151,1728975600"; d="scan'208";a="118755062" Received: from kkkuntal-desk3 (HELO [10.124.220.196]) ([10.124.220.196]) by orviesa002-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 13 Nov 2024 09:49:01 -0800 Message-ID: <8211a3d2-66e0-4563-b804-8aa4bd0f0de9@intel.com> Date: Wed, 13 Nov 2024 09:49:00 -0800 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 v3 1/1] x86/fred: Clear WFE in missing-ENDBRANCH #CPs To: "Xin Li (Intel)" , 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, andrew.cooper3@citrix.com References: <20241113091313.3717338-1-xin@zytor.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/13/24 01:13, Xin Li (Intel) wrote: > An indirect branch instruction sets the CPU indirect branch tracker > (IBT) into WAIT_FOR_ENDBRANCH (WFE) state, and WFE stays asserted > across the instruction boundary. When decoder finds an instruction > and WFE is set, and the instruction is not the appropriate ENDBR, it > raises a #CP fault. > > For the kernel IBT no ENDBR selftest where #CPs are deliberately > triggerred, the WFE state of the interrupted context needs to be > cleared to let execution continue. Otherwise when the CPU resumes > from the instruction that just caused the previous #CP, another > missing-ENDBRANCH #CP is raised and the CPU enters a dead loop. > > This is not a problem with IDT because it doesn't preserve WFE and > IRET doesn't set WFE. But FRED provides space on the entry stack > (in an expanded CS area) to save and restore the WFE state, thus the > WFE state is no longer clobbered, so software must clear it. > > Clear WFE to avoid dead looping in ibt_clear_fred_wfe() and the > !ibt_fatal code path when execution is allowed to continue. > > Clobbering WFE in any other circumstance is a security-relevant bug. With the minor fixes Ingo mentioned: Acked-by: Dave Hansen