From e5a3331a2e98401b838f288d5dce51aa907b8fb2 Mon Sep 17 00:00:00 2001 From: Borislav Petkov Date: Tue, 18 Feb 2025 12:13:33 +0100 Subject: x86/bugs: KVM: Add support for SRSO_MSR_FIX commit 8442df2b49ed9bcd67833ad4f091d15ac91efd00 upstream. Add support for CPUID Fn8000_0021_EAX[31] (SRSO_MSR_FIX). If this bit is 1, it indicates that software may use MSR BP_CFG[BpSpecReduce] to mitigate SRSO. Enable BpSpecReduce to mitigate SRSO across guest/host boundaries. Switch back to enabling the bit when virtualization is enabled and to clear the bit when virtualization is disabled because using a MSR slot would clear the bit when the guest is exited and any training the guest has done, would potentially influence the host kernel when execution enters the kernel and hasn't VMRUN the guest yet. More detail on the public thread in Link below. Co-developed-by: Sean Christopherson Signed-off-by: Sean Christopherson Signed-off-by: Borislav Petkov (AMD) Link: https://lore.kernel.org/r/20241202120416.6054-1-bp@kernel.org Signed-off-by: Harshit Mogalapalli Signed-off-by: Greg Kroah-Hartman --- Documentation/admin-guide/hw-vuln/srso.rst | 13 +++++++++++++ 1 file changed, 13 insertions(+) (limited to 'Documentation') diff --git a/Documentation/admin-guide/hw-vuln/srso.rst b/Documentation/admin-guide/hw-vuln/srso.rst index 2ad1c05b8c88..66af95251a3d 100644 --- a/Documentation/admin-guide/hw-vuln/srso.rst +++ b/Documentation/admin-guide/hw-vuln/srso.rst @@ -104,7 +104,20 @@ The possible values in this file are: (spec_rstack_overflow=ibpb-vmexit) + * 'Mitigation: Reduced Speculation': + This mitigation gets automatically enabled when the above one "IBPB on + VMEXIT" has been selected and the CPU supports the BpSpecReduce bit. + + It gets automatically enabled on machines which have the + SRSO_USER_KERNEL_NO=1 CPUID bit. In that case, the code logic is to switch + to the above =ibpb-vmexit mitigation because the user/kernel boundary is + not affected anymore and thus "safe RET" is not needed. + + After enabling the IBPB on VMEXIT mitigation option, the BpSpecReduce bit + is detected (functionality present on all such machines) and that + practically overrides IBPB on VMEXIT as it has a lot less performance + impact and takes care of the guest->host attack vector too. In order to exploit vulnerability, an attacker needs to: -- cgit v1.2.3