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target/i386: kvm: do not access uninitialized variable on older kernels

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KVM support for AMX includes a new system attribute, KVM_X86_XCOMP_GUEST_SUPP.
Commit 19db68ca68 ("x86: Grant AMX permission for guest", 2022-03-15) however
did not fully consider the behavior on older kernels.  First, it warns
too aggressively.  Second, it invokes the KVM_GET_DEVICE_ATTR ioctl
unconditionally and then uses the "bitmask" variable, which remains
uninitialized if the ioctl fails.  Third, kvm_ioctl returns -errno rather
than -1 on errors.

While at it, explain why the ioctl is needed and KVM_GET_SUPPORTED_CPUID
is not enough.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2022-03-18 16:23:47 +01:00
parent e2fb7d8aa2
commit 3ec5ad4008
1 changed files with 13 additions and 4 deletions
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17
target/i386/kvm/kvm.c
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@ -411,6 +411,12 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, uint32_t function,
}
} else if (function == 0xd && index == 0 &&
(reg == R_EAX || reg == R_EDX)) {
/*
* The value returned by KVM_GET_SUPPORTED_CPUID does not include
* features that still have to be enabled with the arch_prctl
* system call. QEMU needs the full value, which is retrieved
* with KVM_GET_DEVICE_ATTR.
*/
struct kvm_device_attr attr = {
.group = 0,
.attr = KVM_X86_XCOMP_GUEST_SUPP,
@ -419,13 +425,16 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, uint32_t function,
bool sys_attr = kvm_check_extension(s, KVM_CAP_SYS_ATTRIBUTES);
if (!sys_attr) {
warn_report("cannot get sys attribute capabilities %d", sys_attr);
return ret;
}
int rc = kvm_ioctl(s, KVM_GET_DEVICE_ATTR, &attr);
if (rc == -1 && (errno == ENXIO || errno == EINVAL)) {
warn_report("KVM_GET_DEVICE_ATTR(0, KVM_X86_XCOMP_GUEST_SUPP) "
"error: %d", rc);
if (rc < 0) {
if (rc != -ENXIO) {
warn_report("KVM_GET_DEVICE_ATTR(0, KVM_X86_XCOMP_GUEST_SUPP) "
"error: %d", rc);
}
return ret;
}
ret = (reg == R_EAX) ? bitmask : bitmask >> 32;
} else if (function == 0x80000001 && reg == R_ECX) {