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|  | 1 | +.. SPDX-License-Identifier: GPL-2.0 | 
|  | 2 | +
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|  | 3 | +VMSCAPE | 
|  | 4 | +======= | 
|  | 5 | + | 
|  | 6 | +VMSCAPE is a vulnerability that may allow a guest to influence the branch | 
|  | 7 | +prediction in host userspace. It particularly affects hypervisors like QEMU. | 
|  | 8 | + | 
|  | 9 | +Even if a hypervisor may not have any sensitive data like disk encryption keys, | 
|  | 10 | +guest-userspace may be able to attack the guest-kernel using the hypervisor as | 
|  | 11 | +a confused deputy. | 
|  | 12 | + | 
|  | 13 | +Affected processors | 
|  | 14 | +------------------- | 
|  | 15 | + | 
|  | 16 | +The following CPU families are affected by VMSCAPE: | 
|  | 17 | + | 
|  | 18 | +**Intel processors:** | 
|  | 19 | +  - Skylake generation (Parts without Enhanced-IBRS) | 
|  | 20 | +  - Cascade Lake generation - (Parts affected by ITS guest/host separation) | 
|  | 21 | +  - Alder Lake and newer (Parts affected by BHI) | 
|  | 22 | + | 
|  | 23 | +Note that, BHI affected parts that use BHB clearing software mitigation e.g. | 
|  | 24 | +Icelake are not vulnerable to VMSCAPE. | 
|  | 25 | + | 
|  | 26 | +**AMD processors:** | 
|  | 27 | +  - Zen series (families 0x17, 0x19, 0x1a) | 
|  | 28 | + | 
|  | 29 | +** Hygon processors:** | 
|  | 30 | + - Family 0x18 | 
|  | 31 | + | 
|  | 32 | +Mitigation | 
|  | 33 | +---------- | 
|  | 34 | + | 
|  | 35 | +Conditional IBPB | 
|  | 36 | +---------------- | 
|  | 37 | + | 
|  | 38 | +Kernel tracks when a CPU has run a potentially malicious guest and issues an | 
|  | 39 | +IBPB before the first exit to userspace after VM-exit. If userspace did not run | 
|  | 40 | +between VM-exit and the next VM-entry, no IBPB is issued. | 
|  | 41 | + | 
|  | 42 | +Note that the existing userspace mitigation against Spectre-v2 is effective in | 
|  | 43 | +protecting the userspace. They are insufficient to protect the userspace VMMs | 
|  | 44 | +from a malicious guest. This is because Spectre-v2 mitigations are applied at | 
|  | 45 | +context switch time, while the userspace VMM can run after a VM-exit without a | 
|  | 46 | +context switch. | 
|  | 47 | + | 
|  | 48 | +Vulnerability enumeration and mitigation is not applied inside a guest. This is | 
|  | 49 | +because nested hypervisors should already be deploying IBPB to isolate | 
|  | 50 | +themselves from nested guests. | 
|  | 51 | + | 
|  | 52 | +SMT considerations | 
|  | 53 | +------------------ | 
|  | 54 | + | 
|  | 55 | +When Simultaneous Multi-Threading (SMT) is enabled, hypervisors can be | 
|  | 56 | +vulnerable to cross-thread attacks. For complete protection against VMSCAPE | 
|  | 57 | +attacks in SMT environments, STIBP should be enabled. | 
|  | 58 | + | 
|  | 59 | +The kernel will issue a warning if SMT is enabled without adequate STIBP | 
|  | 60 | +protection. Warning is not issued when: | 
|  | 61 | + | 
|  | 62 | +- SMT is disabled | 
|  | 63 | +- STIBP is enabled system-wide | 
|  | 64 | +- Intel eIBRS is enabled (which implies STIBP protection) | 
|  | 65 | + | 
|  | 66 | +System information and options | 
|  | 67 | +------------------------------ | 
|  | 68 | + | 
|  | 69 | +The sysfs file showing VMSCAPE mitigation status is: | 
|  | 70 | + | 
|  | 71 | +  /sys/devices/system/cpu/vulnerabilities/vmscape | 
|  | 72 | + | 
|  | 73 | +The possible values in this file are: | 
|  | 74 | + | 
|  | 75 | + * 'Not affected': | 
|  | 76 | + | 
|  | 77 | +   The processor is not vulnerable to VMSCAPE attacks. | 
|  | 78 | + | 
|  | 79 | + * 'Vulnerable': | 
|  | 80 | + | 
|  | 81 | +   The processor is vulnerable and no mitigation has been applied. | 
|  | 82 | + | 
|  | 83 | + * 'Mitigation: IBPB before exit to userspace': | 
|  | 84 | + | 
|  | 85 | +   Conditional IBPB mitigation is enabled. The kernel tracks when a CPU has | 
|  | 86 | +   run a potentially malicious guest and issues an IBPB before the first | 
|  | 87 | +   exit to userspace after VM-exit. | 
|  | 88 | + | 
|  | 89 | + * 'Mitigation: IBPB on VMEXIT': | 
|  | 90 | + | 
|  | 91 | +   IBPB is issued on every VM-exit. This occurs when other mitigations like | 
|  | 92 | +   RETBLEED or SRSO are already issuing IBPB on VM-exit. | 
|  | 93 | + | 
|  | 94 | +Mitigation control on the kernel command line | 
|  | 95 | +---------------------------------------------- | 
|  | 96 | + | 
|  | 97 | +The mitigation can be controlled via the ``vmscape=`` command line parameter: | 
|  | 98 | + | 
|  | 99 | + * ``vmscape=off``: | 
|  | 100 | + | 
|  | 101 | +   Disable the VMSCAPE mitigation. | 
|  | 102 | + | 
|  | 103 | + * ``vmscape=ibpb``: | 
|  | 104 | + | 
|  | 105 | +   Enable conditional IBPB mitigation (default when CONFIG_MITIGATION_VMSCAPE=y). | 
|  | 106 | + | 
|  | 107 | + * ``vmscape=force``: | 
|  | 108 | + | 
|  | 109 | +   Force vulnerability detection and mitigation even on processors that are | 
|  | 110 | +   not known to be affected. | 
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