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 90C79186E2C for ; Wed, 11 Sep 2024 23:44:34 +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=1726098276; cv=none; b=l62C1XB9giDIgV3RmVh+mSt5kEll9k4xp0g96NyI9ptkWuIb4mMVui4GS9YtO0WLFOonp6CIZmJ1iLVcAkvpOExkithKDGm+DL0rZH3CgOTgZbaHv+KuYBcvt84kFOXhPY3oNzhKS+BimA0YUNC3KtOFVQMxCy5m5SZE1M4UAGs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1726098276; c=relaxed/simple; bh=y2VXaeJJxVoGvguqe8WDtlnLzOelU9lMxIjVbY1WZ6k=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=tNOLDk10nUxZxFz0EOQ8p1uf0R7vnJUFvlT0IJ23rIj1/TtmUYL4TyAlvH/T/C7uipdlBOmCwJENnyMrvYYc4XvyhkvuECpwfBsiKaU9H0wEXsnd7whAY2ugQL6EkfaknHyrGFzXf78fMmkZF5sF5xyClVnLrBDuMbHBAtkmbjA= ARC-Authentication-Results:i=1; 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h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=XcEOl5XW9ADrF1o4FFLpCrZ7Pr02ZVSj0744ToAzev7PTHY7c2dJ4VJTkllky/l5y tFJEb22xwyas25ISzHvPfK/VDWIqYYRz83NBK16GhGzpZ320/8MnrSqAQCfbKTxXVl Ex85UTHpjw48N0BV4i7x5U9dw9hhIpt+1ti+Kt5yco6akn8Q6WOv55jE6vZgQ9BBH5 ifGOsXMDqmUdYxrBrvgPrjSfPqkykUs6usGZVQ01EAhWIshNzvw4MMxaNjqhJq3Zm3 UQ1wITxCwYYMH0++9trCG7y/aWRhhPCSnaO+CbP2WkKdurB6fT5fTuMaVqHBd78wZG ky7QMWE5dbxBw== Message-ID: <9efbfb57-d454-4bad-bdb8-e3000b6ba0ce@zytor.com> Date: Wed, 11 Sep 2024 16:44:05 -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; 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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. Yep, it's a much better story including historical issues with IDT. I will use it as the comment above ibt_clear_fred_wfe() in the next iteration, but keep the change log. Is it okay? > > > 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 > 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