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From 688c9f386635544dbc468171a32fbc84f0c9224e Mon Sep 17 00:00:00 2001
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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Fri, 18 Mar 2022 16:23:47 +0100
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Subject: [PATCH 12/24] target/i386: kvm: do not access uninitialized variable
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on older kernels
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RH-Author: Paul Lai <plai@redhat.com>
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RH-MergeRequest: 176: Enable KVM AMX support
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RH-Commit: [12/13] 776fac1e7d1aa16ec5f4d99ddad3039eab8212af
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RH-Bugzilla: 1916415
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RH-Acked-by: Cornelia Huck <cohuck@redhat.com>
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RH-Acked-by: Igor Mammedov <imammedo@redhat.com>
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RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
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KVM support for AMX includes a new system attribute, KVM_X86_XCOMP_GUEST_SUPP.
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Commit 19db68ca68 ("x86: Grant AMX permission for guest", 2022-03-15) however
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did not fully consider the behavior on older kernels. First, it warns
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too aggressively. Second, it invokes the KVM_GET_DEVICE_ATTR ioctl
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unconditionally and then uses the "bitmask" variable, which remains
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uninitialized if the ioctl fails. Third, kvm_ioctl returns -errno rather
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than -1 on errors.
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While at it, explain why the ioctl is needed and KVM_GET_SUPPORTED_CPUID
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is not enough.
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Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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(cherry picked from commit 3ec5ad40081b14af28496198b4d08dbe13386790)
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Signed-off-by: Paul Lai <plai@redhat.com>
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---
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target/i386/kvm/kvm.c | 17 +++++++++++++----
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1 file changed, 13 insertions(+), 4 deletions(-)
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diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
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index b1128b0e07..bd439e56ad 100644
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--- a/target/i386/kvm/kvm.c
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+++ b/target/i386/kvm/kvm.c
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@@ -409,6 +409,12 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, uint32_t function,
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}
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} else if (function == 0xd && index == 0 &&
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(reg == R_EAX || reg == R_EDX)) {
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+ /*
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+ * The value returned by KVM_GET_SUPPORTED_CPUID does not include
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+ * features that still have to be enabled with the arch_prctl
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+ * system call. QEMU needs the full value, which is retrieved
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+ * with KVM_GET_DEVICE_ATTR.
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+ */
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struct kvm_device_attr attr = {
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.group = 0,
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.attr = KVM_X86_XCOMP_GUEST_SUPP,
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@@ -417,13 +423,16 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, uint32_t function,
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bool sys_attr = kvm_check_extension(s, KVM_CAP_SYS_ATTRIBUTES);
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if (!sys_attr) {
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- warn_report("cannot get sys attribute capabilities %d", sys_attr);
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+ return ret;
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}
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int rc = kvm_ioctl(s, KVM_GET_DEVICE_ATTR, &attr);
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- if (rc == -1 && (errno == ENXIO || errno == EINVAL)) {
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- warn_report("KVM_GET_DEVICE_ATTR(0, KVM_X86_XCOMP_GUEST_SUPP) "
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- "error: %d", rc);
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+ if (rc < 0) {
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+ if (rc != -ENXIO) {
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+ warn_report("KVM_GET_DEVICE_ATTR(0, KVM_X86_XCOMP_GUEST_SUPP) "
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+ "error: %d", rc);
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+ }
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+ return ret;
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}
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ret = (reg == R_EAX) ? bitmask : bitmask >> 32;
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} else if (function == 0x80000001 && reg == R_ECX) {
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--
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2.35.3
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