From nobody Sat Nov 23 22:18:57 2024 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 07ADC1A0B13; Mon, 11 Nov 2024 16:39:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1731343177; cv=none; b=NGYos0ttCALpIfaOnnEa7LX0RUKcpZZq7Ke+lVkhfiUBQtQNYkvHnUx58m8KAL2I2gUBebp3ZsYOp9DMRHlQgSMQ4eX6M5lsLv25uoQf8Cb34W/+0kxVsfxa+nXdSj2Xi83FQXr3BgXDJacUrWDJMOlPMGnhpRVBKVe/0cqYC14= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1731343177; c=relaxed/simple; bh=7HDQrOeHzesoq08mXwj6oE4jK5rhJKQzrH8b34Uv2k4=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=CR45blTQ0kgn7zZ2Lxk44H02RIFFxtgGYfO5o5pw0k/ywLrjXCzr14NKWHK59NbLYZrUicc8lSEdVjUrF0tKhIiY69/ToRbiqrrgYdqsXX3m+Aqzw3f+qGjkjJZQbPWcDTfgJ885K7we7rxOlhKLZPgG8Y0OVixlxpSEUJLzcSI= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=BB5+8ioJ; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="BB5+8ioJ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8C122C4CED7; Mon, 11 Nov 2024 16:39:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1731343176; bh=7HDQrOeHzesoq08mXwj6oE4jK5rhJKQzrH8b34Uv2k4=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=BB5+8ioJvp+yzp8h3GJLoj/X/aCEDdrslkwehkNhxnVwIQp17Hm7Of1tA+qdosGGp nd4WKni+7IDBguleEa4dPdzpwmpyxUBtblng3ITt2X2ASMMVLSq1vhT0ubeYP6H6m+ qVOeFANgJWoQmKPRTFr5uhWu1rMj5QNsEz6HCYFfcNizBr1G6NnV455mAq1tqp/+KJ jHofPd164o6IoPytkZKX+ZKLdVHuiCK22nBzmQegCLGAZ0pqF6dEZfzxZwp8wSwX+u o3/OjESIAZ+7aXuOVSjx6AId92dnrZluHkFAaV3D7uAKt0FMOjh11MCqdRV5T1kMhw jKAO/gcTzLMtA== From: Amit Shah To: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, x86@kernel.org, linux-doc@vger.kernel.org Cc: amit.shah@amd.com, thomas.lendacky@amd.com, bp@alien8.de, tglx@linutronix.de, peterz@infradead.org, jpoimboe@kernel.org, pawan.kumar.gupta@linux.intel.com, corbet@lwn.net, mingo@redhat.com, dave.hansen@linux.intel.com, hpa@zytor.com, seanjc@google.com, pbonzini@redhat.com, daniel.sneddon@linux.intel.com, kai.huang@intel.com, sandipan.das@amd.com, boris.ostrovsky@oracle.com, Babu.Moger@amd.com, david.kaplan@amd.com, dwmw@amazon.co.uk, andrew.cooper3@citrix.com Subject: [RFC PATCH v2 1/3] x86: cpu/bugs: update SpectreRSB comments for AMD Date: Mon, 11 Nov 2024 17:39:11 +0100 Message-ID: <20241111163913.36139-2-amit@kernel.org> X-Mailer: git-send-email 2.47.0 In-Reply-To: <20241111163913.36139-1-amit@kernel.org> References: <20241111163913.36139-1-amit@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" From: Amit Shah AMD CPUs do not fall back to the BTB when the RSB underflows for RET address speculation. AMD CPUs have not needed to stuff the RSB for underflow conditions. The RSB poisoning case is addressed by RSB filling - clean up the FIXME comment about it. Signed-off-by: Amit Shah --- arch/x86/kernel/cpu/bugs.c | 5 ----- 1 file changed, 5 deletions(-) diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index 47a01d4028f6..0aa629b5537d 100644 --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -1828,9 +1828,6 @@ static void __init spectre_v2_select_mitigation(void) * speculated return targets may come from the branch predictor, * which could have a user-poisoned BTB or BHB entry. * - * AMD has it even worse: *all* returns are speculated from the BTB, - * regardless of the state of the RSB. - * * When IBRS or eIBRS is enabled, the "user -> kernel" attack * scenario is mitigated by the IBRS branch prediction isolation * properties, so the RSB buffer filling wouldn't be necessary to @@ -1852,8 +1849,6 @@ static void __init spectre_v2_select_mitigation(void) * * So to mitigate all cases, unconditionally fill RSB on context * switches. - * - * FIXME: Is this pointless for retbleed-affected AMD? */ setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW); pr_info("Spectre v2 / SpectreRSB mitigation: Filling RSB on context switc= h\n"); --=20 2.47.0