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[85.187.216.236]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-b7654d502cfsm469617166b.19.2025.11.21.06.23.57 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 21 Nov 2025 06:23:58 -0800 (PST) Message-ID: <5cdca004-5228-4f07-b9b8-901880f59bb7@suse.com> Date: Fri, 21 Nov 2025 16:23:56 +0200 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 09/11] x86/vmscape: Deploy BHB clearing mitigation To: Pawan Gupta , x86@kernel.org, David Kaplan , "H. Peter Anvin" , Josh Poimboeuf , Sean Christopherson , Paolo Bonzini , Borislav Petkov , Dave Hansen Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Asit Mallick , Tao Zhang References: <20251119-vmscape-bhb-v4-0-1adad4e69ddc@linux.intel.com> <20251119-vmscape-bhb-v4-9-1adad4e69ddc@linux.intel.com> From: Nikolay Borisov Content-Language: en-US Autocrypt: addr=nik.borisov@suse.com; keydata= xsFNBGcrpvIBEAD5cAR5+qu30GnmPrK9veWX5RVzzbgtkk9C/EESHy9Yz0+HWgCVRoNyRQsZ 7DW7vE1KhioDLXjDmeu8/0A8u5nFMqv6d1Gt1lb7XzSAYw7uSWXLPEjFBtz9+fBJJLgbYU7G OpTKy6gRr6GaItZze+r04PGWjeyVUuHZuncTO7B2huxcwIk9tFtRX21gVSOOC96HcxSVVA7X N/LLM2EOL7kg4/yDWEhAdLQDChswhmdpHkp5g6ytj9TM8bNlq9I41hl/3cBEeAkxtb/eS5YR 88LBb/2FkcGnhxkGJPNB+4Siku7K8Mk2Y6elnkOctJcDvk29DajYbQnnW4nhfelZuLNupb1O M0912EvzOVI0dIVgR+xtosp66bYTOpX4Xb0fylED9kYGiuEAeoQZaDQ2eICDcHPiaLzh+6cc pkVTB0sXkWHUsPamtPum6/PgWLE9vGI5s+FaqBaqBYDKyvtJfLK4BdZng0Uc3ijycPs3bpbQ bOnK9LD8TYmYaeTenoNILQ7Ut54CCEXkP446skUMKrEo/HabvkykyWqWiIE/UlAYAx9+Ckho TT1d2QsmsAiYYWwjU8igXBecIbC0uRtF/cTfelNGrQwbICUT6kJjcOTpQDaVyIgRSlUMrlNZ XPVEQ6Zq3/aENA8ObhFxE5PLJPizJH6SC89BMKF3zg6SKx0qzQARAQABzSZOaWtvbGF5IEJv cmlzb3YgPG5pay5ib3Jpc292QHN1c2UuY29tPsLBkQQTAQoAOxYhBDuWB8EJLBUZCPjT3SRn XZEnyhfsBQJnK6byAhsDBQsJCAcCAiICBhUKCQgLAgQWAgMBAh4HAheAAAoJECRnXZEnyhfs XbIQAJxuUnelGdXbSbtovBNm+HF3LtT0XnZ0+DoR0DemUGuA1bZAlaOXGr5mvVbTgaoGUQIJ 3Ejx3UBEG7ZSJcfJobB34w1qHEDO0pN9orGIFT9Bic3lqhawD2r85QMcWwjsZH5FhyRx7P2o DTuUClLMO95GuHYQngBF2rHHl8QMJPVKsR18w4IWAhALpEApxa3luyV7pAAqKllfCNt7tmed uKmclf/Sz6qoP75CvEtRbfAOqYgG1Uk9A62C51iAPe35neMre3WGLsdgyMj4/15jPYi+tOUX Tc7AAWgc95LXyPJo8069MOU73htZmgH4OYy+S7f+ArXD7h8lTLT1niff2bCPi6eiAQq6b5CJ Ka4/27IiZo8tm1XjLYmoBmaCovqx5y5Xt2koibIWG3ZGD2I+qRwZ0UohKRH6kKVHGcrmCv0J YO8yIprxgoYmA7gq21BpTqw3D4+8xujn/6LgndLKmGESM1FuY3ymXgj5983eqaxicKpT9iq8 /a1j31tms4azR7+6Dt8H4SagfN6VbJ0luPzobrrNFxUgpjR4ZyQQ++G7oSRdwjfIh1wuCF6/ mDUNcb6/kA0JS9otiC3omfht47yQnvod+MxFk1lTNUu3hePJUwg1vT1te3vO5oln8lkUo9BU knlYpQ7QA2rDEKs+YWqUstr4pDtHzwQ6mo0rqP+zzsFNBGcrpvIBEADGYTFkNVttZkt6e7yA LNkv3Q39zQCt8qe7qkPdlj3CqygVXfw+h7GlcT9fuc4kd7YxFys4/Wd9icj9ZatGMwffONmi LnUotIq2N7+xvc4Xu76wv+QJpiuGEfCDB+VdZOmOzUPlmMkcJc/EDSH4qGogIYRu72uweKEq VfBI43PZIGpGJ7TjS3THX5WVI2YNSmuwqxnQF/iVqDtD2N72ObkBwIf9GnrOgxEyJ/SQq2R0 g7hd6IYk7SOKt1a8ZGCN6hXXKzmM6gHRC8fyWeTqJcK4BKSdX8PzEuYmAJjSfx4w6DoxdK5/ 9sVrNzaVgDHS0ThH/5kNkZ65KNR7K2nk45LT5Crjbg7w5/kKDY6/XiXDx7v/BOR/a+Ryo+lM MffN3XSnAex8cmIhNINl5Z8CAvDLUtItLcbDOv7hdXt6DSyb65CdyY8JwOt6CWno1tdjyDEG 5ANwVPYY878IFkOJLRTJuUd5ltybaSWjKIwjYJfIXuoyzE7OL63856MC/Os8PcLfY7vYY2LB cvKH1qOcs+an86DWX17+dkcKD/YLrpzwvRMur5+kTgVfXcC0TAl39N4YtaCKM/3ugAaVS1Mw MrbyGnGqVMqlCpjnpYREzapSk8XxbO2kYRsZQd8J9ei98OSqgPf8xM7NCULd/xaZLJUydql1 JdSREId2C15jut21aQARAQABwsF2BBgBCgAgFiEEO5YHwQksFRkI+NPdJGddkSfKF+wFAmcr pvICGwwACgkQJGddkSfKF+xuuxAA4F9iQc61wvAOAidktv4Rztn4QKy8TAyGN3M8zYf/A5Zx VcGgX4J4MhRUoPQNrzmVlrrtE2KILHxQZx5eQyPgixPXri42oG5ePEXZoLU5GFRYSPjjTYmP ypyTPN7uoWLfw4TxJqWCGRLsjnkwvyN3R4161Dty4Uhzqp1IkNhl3ifTDYEvbnmHaNvlvvna 7+9jjEBDEFYDMuO/CA8UtoVQXjy5gtOhZZkEsptfwQYc+E9U99yxGofDul7xH41VdXGpIhUj 4wjd3IbgaCiHxxj/M9eM99ybu5asvHyMo3EFPkyWxZsBlUN/riFXGspG4sT0cwOUhG2ZnExv XXhOGKs/y3VGhjZeCDWZ+0ZQHPCL3HUebLxW49wwLxvXU6sLNfYnTJxdqn58Aq4sBXW5Un0Q vfbd9VFV/bKFfvUscYk2UKPi9vgn1hY38IfmsnoS8b0uwDq75IBvup9pYFyNyPf5SutxhFfP JDjakbdjBoYDWVoaPbp5KAQ2VQRiR54lir/inyqGX+dwzPX/F4OHfB5RTiAFLJliCxniKFsM d8eHe88jWjm6/ilx4IlLl9/MdVUGjLpBi18X7ejLz3U2quYD8DBAGzCjy49wJ4Di4qQjblb2 pTXoEyM2L6E604NbDu0VDvHg7EXh1WwmijEu28c/hEB6DwtzslLpBSsJV0s1/jE= In-Reply-To: <20251119-vmscape-bhb-v4-9-1adad4e69ddc@linux.intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 11/20/25 08:19, Pawan Gupta wrote: > IBPB mitigation for VMSCAPE is an overkill on CPUs that are only affected > by the BHI variant of VMSCAPE. On such CPUs, eIBRS already provides > indirect branch isolation between guest and host userspace. However, branch > history from guest may also influence the indirect branches in host > userspace. > > To mitigate the BHI aspect, use clear_bhb_loop(). > > Signed-off-by: Pawan Gupta > --- > Documentation/admin-guide/hw-vuln/vmscape.rst | 4 ++++ > arch/x86/include/asm/nospec-branch.h | 2 ++ > arch/x86/kernel/cpu/bugs.c | 30 ++++++++++++++++++++------- > 3 files changed, 29 insertions(+), 7 deletions(-) > > diff --git a/Documentation/admin-guide/hw-vuln/vmscape.rst b/Documentation/admin-guide/hw-vuln/vmscape.rst > index d9b9a2b6c114c05a7325e5f3c9d42129339b870b..dc63a0bac03d43d1e295de0791dd6497d101f986 100644 > --- a/Documentation/admin-guide/hw-vuln/vmscape.rst > +++ b/Documentation/admin-guide/hw-vuln/vmscape.rst > @@ -86,6 +86,10 @@ The possible values in this file are: > run a potentially malicious guest and issues an IBPB before the first > exit to userspace after VM-exit. > > + * 'Mitigation: Clear BHB before exit to userspace': > + > + As above, conditional BHB clearing mitigation is enabled. > + > * 'Mitigation: IBPB on VMEXIT': > > IBPB is issued on every VM-exit. This occurs when other mitigations like > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h > index 15a2fa8f2f48a066e102263513eff9537ac1d25f..1e8c26c37dbed4256b35101fb41c0e1eb6ef9272 100644 > --- a/arch/x86/include/asm/nospec-branch.h > +++ b/arch/x86/include/asm/nospec-branch.h > @@ -388,6 +388,8 @@ extern void write_ibpb(void); > > #ifdef CONFIG_X86_64 > extern void clear_bhb_loop(void); > +#else > +static inline void clear_bhb_loop(void) {} > #endif > > extern void (*x86_return_thunk)(void); > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c > index cbb3341b9a19f835738eda7226323d88b7e41e52..d12c07ccf59479ecf590935607394492c988b2ff 100644 > --- a/arch/x86/kernel/cpu/bugs.c > +++ b/arch/x86/kernel/cpu/bugs.c > @@ -109,9 +109,8 @@ DEFINE_PER_CPU(u64, x86_spec_ctrl_current); > EXPORT_PER_CPU_SYMBOL_GPL(x86_spec_ctrl_current); > > /* > - * Set when the CPU has run a potentially malicious guest. An IBPB will > - * be needed to before running userspace. That IBPB will flush the branch > - * predictor content. > + * Set when the CPU has run a potentially malicious guest. Indicates that a > + * branch predictor flush is needed before running userspace. > */ > DEFINE_PER_CPU(bool, x86_predictor_flush_exit_to_user); > EXPORT_PER_CPU_SYMBOL_GPL(x86_predictor_flush_exit_to_user); > @@ -3200,13 +3199,15 @@ enum vmscape_mitigations { > VMSCAPE_MITIGATION_AUTO, > VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER, > VMSCAPE_MITIGATION_IBPB_ON_VMEXIT, > + VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER, > }; > > static const char * const vmscape_strings[] = { > - [VMSCAPE_MITIGATION_NONE] = "Vulnerable", > + [VMSCAPE_MITIGATION_NONE] = "Vulnerable", > /* [VMSCAPE_MITIGATION_AUTO] */ > - [VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER] = "Mitigation: IBPB before exit to userspace", > - [VMSCAPE_MITIGATION_IBPB_ON_VMEXIT] = "Mitigation: IBPB on VMEXIT", > + [VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER] = "Mitigation: IBPB before exit to userspace", > + [VMSCAPE_MITIGATION_IBPB_ON_VMEXIT] = "Mitigation: IBPB on VMEXIT", > + [VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER] = "Mitigation: Clear BHB before exit to userspace", > }; > > static enum vmscape_mitigations vmscape_mitigation __ro_after_init = > @@ -3253,8 +3254,19 @@ static void __init vmscape_select_mitigation(void) > vmscape_mitigation = VMSCAPE_MITIGATION_NONE; > break; > > + case VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER: > + if (!boot_cpu_has(X86_FEATURE_BHI_CTRL)) > + vmscape_mitigation = VMSCAPE_MITIGATION_NONE; > + break; Am I missing something or this case can never execute because VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER is only ever set if mitigation is VMSCAPE_MITIGATION_AUTO in the below branch? Perhaps just remove it? This just shows how confusing the logic for choosing the mitigations has become.... > case VMSCAPE_MITIGATION_AUTO: > - if (boot_cpu_has(X86_FEATURE_IBPB)) > + /* > + * CPUs with BHI_CTRL(ADL and newer) can avoid the IBPB and use BHB > + * clear sequence. These CPUs are only vulnerable to the BHI variant > + * of the VMSCAPE attack and does not require an IBPB flush. > + */ > + if (boot_cpu_has(X86_FEATURE_BHI_CTRL)) > + vmscape_mitigation = VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER; > + else if (boot_cpu_has(X86_FEATURE_IBPB)) > vmscape_mitigation = VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER; > else > vmscape_mitigation = VMSCAPE_MITIGATION_NONE;