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authorPaolo Bonzini <pbonzini@redhat.com>2024-07-17 13:04:48 -0400
committerPaolo Bonzini <pbonzini@redhat.com>2024-07-26 14:46:14 -0400
commit5932ca411e533e7ad2b97c47b4357a05fa06c2a5 (patch)
tree7a164c01d4fb3b3f840d9b464d7685c9ba109a76 /Documentation
parentc2adcf051be01d3da1e138c178a680514299461b (diff)
downloadlwn-5932ca411e533e7ad2b97c47b4357a05fa06c2a5.tar.gz
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KVM: x86: disallow pre-fault for SNP VMs before initialization
KVM_PRE_FAULT_MEMORY for an SNP guest can race with sev_gmem_post_populate() in bad ways. The following sequence for instance can potentially trigger an RMP fault: thread A, sev_gmem_post_populate: called thread B, sev_gmem_prepare: places below 'pfn' in a private state in RMP thread A, sev_gmem_post_populate: *vaddr = kmap_local_pfn(pfn + i); thread A, sev_gmem_post_populate: copy_from_user(vaddr, src + i * PAGE_SIZE, PAGE_SIZE); RMP #PF Fix this by only allowing KVM_PRE_FAULT_MEMORY to run after a guest's initial private memory contents have been finalized via KVM_SEV_SNP_LAUNCH_FINISH. Beyond fixing this issue, it just sort of makes sense to enforce this, since the KVM_PRE_FAULT_MEMORY documentation states: "KVM maps memory as if the vCPU generated a stage-2 read page fault" which sort of implies we should be acting on the same guest state that a vCPU would see post-launch after the initial guest memory is all set up. Co-developed-by: Michael Roth <michael.roth@amd.com> Signed-off-by: Michael Roth <michael.roth@amd.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/virt/kvm/api.rst6
1 files changed, 6 insertions, 0 deletions
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index ec1cd8aa1d56..7b512286f8d2 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6402,6 +6402,12 @@ for the current vCPU state. KVM maps memory as if the vCPU generated a
stage-2 read page fault, e.g. faults in memory as needed, but doesn't break
CoW. However, KVM does not mark any newly created stage-2 PTE as Accessed.
+In the case of confidential VM types where there is an initial set up of
+private guest memory before the guest is 'finalized'/measured, this ioctl
+should only be issued after completing all the necessary setup to put the
+guest into a 'finalized' state so that the above semantics can be reliably
+ensured.
+
In some cases, multiple vCPUs might share the page tables. In this
case, the ioctl can be called in parallel.