From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 974CCC83F1A for ; Wed, 23 Jul 2025 09:27:29 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ueVkT-0006eX-Dx; Wed, 23 Jul 2025 05:26:54 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ueVkJ-0006cb-Or for qemu-devel@nongnu.org; Wed, 23 Jul 2025 05:26:44 -0400 Received: from casper.infradead.org ([2001:8b0:10b:1236::1]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ueVkG-0004pJ-Up for qemu-devel@nongnu.org; Wed, 23 Jul 2025 05:26:43 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=MIME-Version:Content-Type:References: In-Reply-To:Date:Cc:To:From:Subject:Message-ID:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=m4Uefa5H3A4n5gzFW2AAQ/fhByYNXLclEZNk/kLu6VA=; b=jwf8YDqjsSePT9oreMuxigrarH btC+CQ7+9qKoPPZ5NG8tA8fvcBHTQEV0CApZR98tqEFFh7eQy/5lyUMVXwac0BHIlfXyaxELP16U7 zq3NW8lmuJRJPNK+rtF19xNvMtsgf8WBAwkFlMthIfyXi4rBq28TjTsb21CoICY1FWIwVJTLqBWsl v3n/LrKpsi7qQGP+XwYw87d7fKGjOAkoNcD5JQwmiGa8JelNoBYjp6NI6dKBPv3rrUHeshzHVttoU Ct5sEBE03ySvDb//2IxyYiplGtTTBmQ2fRrDb5FJfsKq+UdVf0FNlDk+rfylHSbPKXwtku8AfQUIO QiF+e0gA==; Received: from [54.239.6.190] (helo=edge-cache-169.e-lhr50.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1ueVk9-00000008NEA-0gmf; Wed, 23 Jul 2025 09:26:33 +0000 Message-ID: <271351582cbe88731098bd4fbdd8f7ef522f20f6.camel@infradead.org> Subject: Re: [PATCH] i386/kvm: Disable hypercall patching quirk by default From: David Woodhouse To: Mathias Krause , qemu-devel@nongnu.org Cc: Paolo Bonzini , Marcelo Tosatti , kvm@vger.kernel.org, Oliver Upton , Sean Christopherson Date: Wed, 23 Jul 2025 11:26:32 +0200 In-Reply-To: <20250619194204.1089048-1-minipli@grsecurity.net> References: <20250619194204.1089048-1-minipli@grsecurity.net> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-NlCc2A7IKFBP27mSoo8W" User-Agent: Evolution 3.52.3-0ubuntu1 MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by casper.infradead.org. See http://www.infradead.org/rpr.html Received-SPF: none client-ip=2001:8b0:10b:1236::1; envelope-from=BATV+a86420b54da33ea93a81+8004+infradead.org+dwmw2@casper.srs.infradead.org; helo=casper.infradead.org X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, SPF_HELO_NONE=0.001, SPF_NONE=0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org --=-NlCc2A7IKFBP27mSoo8W Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2025-06-19 at 21:42 +0200, Mathias Krause wrote: > KVM has a weird behaviour when a guest executes VMCALL on an AMD system > or VMMCALL on an Intel CPU. Both naturally generate an invalid opcode > exception (#UD) as they are just the wrong instruction for the CPU > given. But instead of forwarding the exception to the guest, KVM tries > to patch the guest instruction to match the host's actual hypercall > instruction. That is doomed to fail as read-only code is rather the > standard these days. But, instead of letting go the patching attempt and > falling back to #UD injection, KVM injects the page fault instead. >=20 > That's wrong on multiple levels. Not only isn't that a valid exception > to be generated by these instructions, confusing attempts to handle > them. It also destroys guest state by doing so, namely the value of CR2. >=20 > Sean attempted to fix that in KVM[1] but the patch was never applied. >=20 > Later, Oliver added a quirk bit in [2] so the behaviour can, at least, > conceptually be disabled. Paolo even called out to add this very > functionality to disable the quirk in QEMU[3]. So lets just do it. >=20 > A new property 'hypercall-patching=3Don|off' is added, for the very > unlikely case that there are setups that really need the patching. > However, these would be vulnerable to memory corruption attacks freely > overwriting code as they please. So, my guess is, there are exactly 0 > systems out there requiring this quirk. I am always wary of making assumptions about how guests behave in the general case. Every time we do so, we seem to find that *some* ancient version of some random network applicance =E2=80=94 or FreeBSD =E2=80=94 do= es exactly the thing we considered unlikely. And customers get sad. As a general rule, before disabling a thing that even *might* have worked for a guest, I'd like to run in a 'warning' mode first. Only after running the whole fleet with such a warning and observing that it *doesn't* trigger, can we actually switch the thing *off*. Can we have 'hypercall-patching=3Don|off|log' ?=20 --=-NlCc2A7IKFBP27mSoo8W Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCD9Aw ggSOMIIDdqADAgECAhAOmiw0ECVD4cWj5DqVrT9PMA0GCSqGSIb3DQEBCwUAMGUxCzAJBgNVBAYT AlVTMRUwEwYDVQQKEwxEaWdpQ2VydCBJbmMxGTAXBgNVBAsTEHd3dy5kaWdpY2VydC5jb20xJDAi BgNVBAMTG0RpZ2lDZXJ0IEFzc3VyZWQgSUQgUm9vdCBDQTAeFw0yNDAxMzAwMDAwMDBaFw0zMTEx MDkyMzU5NTlaMEExCzAJBgNVBAYTAkFVMRAwDgYDVQQKEwdWZXJva2V5MSAwHgYDVQQDExdWZXJv a2V5IFNlY3VyZSBFbWFpbCBHMjCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAMjvgLKj jfhCFqxYyRiW8g3cNFAvltDbK5AzcOaR7yVzVGadr4YcCVxjKrEJOgi7WEOH8rUgCNB5cTD8N/Et GfZI+LGqSv0YtNa54T9D1AWJy08ZKkWvfGGIXN9UFAPMJ6OLLH/UUEgFa+7KlrEvMUupDFGnnR06 aDJAwtycb8yXtILj+TvfhLFhafxroXrflspavejQkEiHjNjtHnwbZ+o43g0/yxjwnarGI3kgcak7 nnI9/8Lqpq79tLHYwLajotwLiGTB71AGN5xK+tzB+D4eN9lXayrjcszgbOv2ZCgzExQUAIt98mre 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