Merge tag 'vmscape-for-linus-20250904' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull vmescape mitigation fixes from Dave Hansen:
 "Mitigate vmscape issue with indirect branch predictor flushes.

  vmscape is a vulnerability that essentially takes Spectre-v2 and
  attacks host userspace from a guest. It particularly affects
  hypervisors like QEMU.

  Even if a hypervisor may not have any sensitive data like disk
  encryption keys, guest-userspace may be able to attack the
  guest-kernel using the hypervisor as a confused deputy.

  There are many ways to mitigate vmscape using the existing Spectre-v2
  defenses like IBRS variants or the IBPB flushes. This series focuses
  solely on IBPB because it works universally across vendors and all
  vulnerable processors. Further work doing vendor and model-specific
  optimizations can build on top of this if needed / wanted.

  Do the normal issue mitigation dance:

   - Add the CPU bug boilerplate

   - Add a list of vulnerable CPUs

   - Use IBPB to flush the branch predictors after running guests"

* tag 'vmscape-for-linus-20250904' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/vmscape: Add old Intel CPUs to affected list
  x86/vmscape: Warn when STIBP is disabled with SMT
  x86/bugs: Move cpu_bugs_smt_update() down
  x86/vmscape: Enable the mitigation
  x86/vmscape: Add conditional IBPB mitigation
  x86/vmscape: Enumerate VMSCAPE bug
  Documentation/hw-vuln: Add VMSCAPE documentation

3 files changed, 122 insertions, 0 deletions

diff --git a/Documentation/admin-guide/hw-vuln/index.rst b/Documentation/admin-guide/hw-vuln/index.rst
index 89ca636081b7..55d747511f83 100644
--- a/Documentation/admin-guide/hw-vuln/index.rst
+++ b/Documentation/admin-guide/hw-vuln/index.rst
@@ -26,3 +26,4 @@ are configurable at compile, boot or run time.
    rsb
    old_microcode
    indirect-target-selection
+   vmscape
diff --git a/Documentation/admin-guide/hw-vuln/vmscape.rst b/Documentation/admin-guide/hw-vuln/vmscape.rst
new file mode 100644
index 000000000000..d9b9a2b6c114
--- /dev/null
+++ b/Documentation/admin-guide/hw-vuln/vmscape.rst
@@ -0,0 +1,110 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+VMSCAPE
+=======
+
+VMSCAPE is a vulnerability that may allow a guest to influence the branch
+prediction in host userspace. It particularly affects hypervisors like QEMU.
+
+Even if a hypervisor may not have any sensitive data like disk encryption keys,
+guest-userspace may be able to attack the guest-kernel using the hypervisor as
+a confused deputy.
+
+Affected processors
+-------------------
+
+The following CPU families are affected by VMSCAPE:
+
+**Intel processors:**
+  - Skylake generation (Parts without Enhanced-IBRS)
+  - Cascade Lake generation - (Parts affected by ITS guest/host separation)
+  - Alder Lake and newer (Parts affected by BHI)
+
+Note that, BHI affected parts that use BHB clearing software mitigation e.g.
+Icelake are not vulnerable to VMSCAPE.
+
+**AMD processors:**
+  - Zen series (families 0x17, 0x19, 0x1a)
+
+** Hygon processors:**
+ - Family 0x18
+
+Mitigation
+----------
+
+Conditional IBPB
+----------------
+
+Kernel tracks when a CPU has run a potentially malicious guest and issues an
+IBPB before the first exit to userspace after VM-exit. If userspace did not run
+between VM-exit and the next VM-entry, no IBPB is issued.
+
+Note that the existing userspace mitigation against Spectre-v2 is effective in
+protecting the userspace. They are insufficient to protect the userspace VMMs
+from a malicious guest. This is because Spectre-v2 mitigations are applied at
+context switch time, while the userspace VMM can run after a VM-exit without a
+context switch.
+
+Vulnerability enumeration and mitigation is not applied inside a guest. This is
+because nested hypervisors should already be deploying IBPB to isolate
+themselves from nested guests.
+
+SMT considerations
+------------------
+
+When Simultaneous Multi-Threading (SMT) is enabled, hypervisors can be
+vulnerable to cross-thread attacks. For complete protection against VMSCAPE
+attacks in SMT environments, STIBP should be enabled.
+
+The kernel will issue a warning if SMT is enabled without adequate STIBP
+protection. Warning is not issued when:
+
+- SMT is disabled
+- STIBP is enabled system-wide
+- Intel eIBRS is enabled (which implies STIBP protection)
+
+System information and options
+------------------------------
+
+The sysfs file showing VMSCAPE mitigation status is:
+
+  /sys/devices/system/cpu/vulnerabilities/vmscape
+
+The possible values in this file are:
+
+ * 'Not affected':
+
+   The processor is not vulnerable to VMSCAPE attacks.
+
+ * 'Vulnerable':
+
+   The processor is vulnerable and no mitigation has been applied.
+
+ * 'Mitigation: IBPB before exit to userspace':
+
+   Conditional IBPB mitigation is enabled. The kernel tracks when a CPU has
+   run a potentially malicious guest and issues an IBPB before the first
+   exit to userspace after VM-exit.
+
+ * 'Mitigation: IBPB on VMEXIT':
+
+   IBPB is issued on every VM-exit. This occurs when other mitigations like
+   RETBLEED or SRSO are already issuing IBPB on VM-exit.
+
+Mitigation control on the kernel command line
+----------------------------------------------
+
+The mitigation can be controlled via the ``vmscape=`` command line parameter:
+
+ * ``vmscape=off``:
+
+   Disable the VMSCAPE mitigation.
+
+ * ``vmscape=ibpb``:
+
+   Enable conditional IBPB mitigation (default when CONFIG_MITIGATION_VMSCAPE=y).
+
+ * ``vmscape=force``:
+
+   Force vulnerability detection and mitigation even on processors that are
+   not known to be affected.
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 747a55abf494..5a7a83c411e9 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3829,6 +3829,7 @@
 					       srbds=off [X86,INTEL]
 					       ssbd=force-off [ARM64]
 					       tsx_async_abort=off [X86]
+					       vmscape=off [X86]
 
 				Exceptions:
 					       This does not have any effect on
@@ -8041,6 +8042,16 @@
 	vmpoff=		[KNL,S390] Perform z/VM CP command after power off.
 			Format: <command>
 
+	vmscape=	[X86] Controls mitigation for VMscape attacks.
+			VMscape attacks can leak information from a userspace
+			hypervisor to a guest via speculative side-channels.
+
+			off		- disable the mitigation
+			ibpb		- use Indirect Branch Prediction Barrier
+					  (IBPB) mitigation (default)
+			force		- force vulnerability detection even on
+					  unaffected processors
+
 	vsyscall=	[X86-64,EARLY]
 			Controls the behavior of vsyscalls (i.e. calls to
 			fixed addresses of 0xffffffffff600x00 from legacy