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Sean Christopherson ce904c8a5b KVM: nSVM: Remove a user-triggerable WARN on nested_svm_load_cr3() succeeding 
[ Upstream commit fc3ba56385 ]

Drop the WARN in svm_set_nested_state() on nested_svm_load_cr3() failing
as it is trivially easy to trigger from userspace by modifying CPUID after
loading CR3.  E.g. modifying the state restoration selftest like so:

  --- tools/testing/selftests/kvm/x86/state_test.c
  +++ tools/testing/selftests/kvm/x86/state_test.c
  @@ -280,7 +280,16 @@ int main(int argc, char *argv[])

                 /* Restore state in a new VM.  */
                  vcpu = vm_recreate_with_one_vcpu(vm);
  -               vcpu_load_state(vcpu, state);
  +
  +               if (stage == 4) {
  +                       state->sregs.cr3 = BIT(44);
  +                       vcpu_load_state(vcpu, state);
  +
  +                       vcpu_set_cpuid_property(vcpu, X86_PROPERTY_MAX_PHY_ADDR, 36);
  +                       __vcpu_nested_state_set(vcpu, &state->nested);
  +               } else {
  +                       vcpu_load_state(vcpu, state);
  +               }

                  /*
                   * Restore XSAVE state in a dummy vCPU, first without doing

generates:

  WARNING: CPU: 30 PID: 938 at arch/x86/kvm/svm/nested.c:1877 svm_set_nested_state+0x34a/0x360 [kvm_amd]
  Modules linked in: kvm_amd kvm irqbypass [last unloaded: kvm]
  CPU: 30 UID: 1000 PID: 938 Comm: state_test Tainted: G        W           6.18.0-rc7-58e10b63777d-next-vm
  Tainted: [W]=WARN
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:svm_set_nested_state+0x34a/0x360 [kvm_amd]
  Call Trace:
   <TASK>
   kvm_arch_vcpu_ioctl+0xf33/0x1700 [kvm]
   kvm_vcpu_ioctl+0x4e6/0x8f0 [kvm]
   __x64_sys_ioctl+0x8f/0xd0
   do_syscall_64+0x61/0xad0
   entry_SYSCALL_64_after_hwframe+0x4b/0x53

Simply delete the WARN instead of trying to prevent userspace from shoving
"illegal" state into CR3.  For better or worse, KVM's ABI allows userspace
to set CPUID after SREGS, and vice versa, and KVM is very permissive when
it comes to guest CPUID.  I.e. attempting to enforce the virtual CPU model
when setting CPUID could break userspace.  Given that the WARN doesn't
provide any meaningful protection for KVM or benefit for userspace, simply
drop it even though the odds of breaking userspace are minuscule.

Opportunistically delete a spurious newline.

Fixes: b222b0b881 ("KVM: nSVM: refactor the CR3 reload on migration")
Cc: stable@vger.kernel.org
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251216161755.1775409-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2026-03-04 07:21:39 -05:00
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README

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.