From 3ec5ad40081b14af28496198b4d08dbe13386790 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Fri, 18 Mar 2022 16:23:47 +0100 Subject: [PATCH] target/i386: kvm: do not access uninitialized variable on older kernels 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 --- target/i386/kvm/kvm.c | 17 +++++++++++++---- 1 file changed, 13 insertions(+), 4 deletions(-) diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c index ef2c68a6f4..06901c2a43 100644 --- a/target/i386/kvm/kvm.c +++ b/target/i386/kvm/kvm.c @@ -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) {