high CVSS 7.8

CVE-2021-46978·Kernel vulnerability

In the Linux kernel, the following vulnerability has been resolved: KVM: nVMX: Always make an attempt to map eVMCS after migration When enlightened VMCS is in use and nested state is migrated with vmx_get_nested_state()/vmx_set_nested_state() KVM can't map evmcs page right away: evmcs gpa is not 'struct kvm_vmx_nested_state_hdr' and we can't read it from VP assist page because userspace may decide to restore HV_X64_MSR_VP_ASSIST_PAGE after restoring nested state (and QEMU, for example, does exactly that). To make sure eVMCS is mapped /vmx_set_nested_state() raises KVM_REQ_GET_NESTED_STATE_PAGES request. Commit f2c7ef3ba955 ("KVM: nSVM: cancel KVM_REQ_GET_NESTED_STATE_PAGES on nested vmexit") added KVM_REQ_GET_NESTED_STATE_PAGES clearing to nested_vmx_vmexit() to make sure MSR permission bitmap is not switched when an immediate exit from L2 to L1 happens right after migration (caused by a pending event, for example). Unfortunately, in the exact same situation we still need to have eVMCS mapped so nested_sync_vmcs12_to_shadow() reflects changes in VMCS12 to eVMCS. As a band-aid, restore nested_get_evmcs_page() when clearing KVM_REQ_GET_NESTED_STATE_PAGES in nested_vmx_vmexit(). The 'fix' is far from being ideal as we can't easily propagate possible failures and even if we could, this is most likely already too late to do so. The whole 'KVM_REQ_GET_NESTED_STATE_PAGES' idea for mapping eVMCS after migration seems to be fragile as we diverge too much from the 'native' path when vmptr loading happens on vmx_set_nested_state().

Severity
high
Software
Kernel
Fixed in
5.12.5
Published
2024-02-28

Affected versions

From: 5.12

Until: 5.12.5

Fixed in: 5.12.5

How to fix this CVE

Update your Linux kernel to version 5.12.5 or later to resolve a critical nested virtualization issue affecting enlightened VMCS (eVMCS) mapping during VM migration. This vulnerability can cause incorrect VMCS12 state synchronization in nested hypervisor environments, leading to privilege escalation or data corruption. Apply the kernel update immediately on systems running KVM with nested virtualization enabled.

sudo dnf update kernel

Defensia detects this vulnerability

How to check if you are affected

  1. Step 1: Check your current kernel version with `uname -r` and compare against 5.12.5 — if below 5.12.5 and kernel version is between 5.12.0-5.12.4, the system is vulnerable.
  2. Step 2: Verify if KVM nested virtualization is enabled by running `cat /sys/module/kvm_intel/parameters/nested` or `cat /sys/module/kvm_amd/parameters/nesting` — if either returns 'Y', the vulnerability is exploitable.
  3. Step 3: Check for active nested VM migrations in /var/log/libvirt/qemu/*.log or journalctl for errors related to 'vmx_set_nested_state', 'eVMCS', or 'KVM_REQ_GET_NESTED_STATE_PAGES' which may indicate exploitation attempts.
  4. Step 4: After patching, run `uname -r` again to confirm kernel version is 5.12.5 or later, then reboot if necessary and re-verify nested VM functionality with a test migration.

FAQ

What is CVE-2021-46978?

CVE-2021-46978 is a kernel vulnerability affecting nested KVM virtualization where enlightened VMCS (eVMCS) fails to map correctly after VM migration, potentially allowing VMCS state desynchronization between nested guests and the hypervisor.

Is CVE-2021-46978 being actively exploited?

No, CVE-2021-46978 is not listed in CISA's known exploited vulnerabilities catalog and no public exploits are available. However, it poses a significant risk in production hypervisor environments due to its high CVSS score of 7.8.

What versions of Kernel are affected by CVE-2021-46978?

Kernel versions 5.12.0 through 5.12.4 are vulnerable. The fix is included in kernel 5.12.5 and all subsequent releases.

How do I check if my server is vulnerable to CVE-2021-46978?

Run `uname -r` to check your kernel version — if it shows 5.12.0 to 5.12.4, you are vulnerable. Additionally, confirm nested KVM is enabled with `cat /sys/module/kvm_intel/parameters/nested` returning 'Y'.

Does Defensia detect CVE-2021-46978?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If the Linux kernel is installed on a monitored server, CVE-2021-46978 will appear in your dashboard with remediation steps.

Related Kernel CVEs

CVE-2024-42256CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: cifs: Fix server re-repick on subrequest retry When a subrequest is marked for needing retry, netfs will call cifs_prepare_write() which will make cifs repick the server for the op before renegotiating credits; it then calls cifs_issue_write() which invokes smb2_async_writev() - which re-repicks the server. If a different server is then selected, this causes the increment of server->in_flight to happen against one record and the decrement to happen against another, leading to misaccounting. Fix this by just removing the repick code in smb2_async_writev(). As this is only called from netfslib-driven code, cifs_prepare_write() should always have been called first, and so server should never be NULL and the preparatory step is repeated in the event that we do a retry. The problem manifests as a warning looking something like: WARNING: CPU: 4 PID: 72896 at fs/smb/client/smb2ops.c:97 smb2_add_credits+0x3f0/0x9e0 [cifs] ... RIP: 0010:smb2_add_credits+0x3f0/0x9e0 [cifs] ... smb2_writev_callback+0x334/0x560 [cifs] cifs_demultiplex_thread+0x77a/0x11b0 [cifs] kthread+0x187/0x1d0 ret_from_fork+0x34/0x60 ret_from_fork_asm+0x1a/0x30 Which may be triggered by a number of different xfstests running against an Azure server in multichannel mode. generic/249 seems the most repeatable, but generic/215, generic/249 and generic/308 may also show it.
CVE-2024-38623CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: fs/ntfs3: Use variable length array instead of fixed size Should fix smatch warning: ntfs_set_label() error: __builtin_memcpy() 'uni->name' too small (20 vs 256)
CVE-2024-36031CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: keys: Fix overwrite of key expiration on instantiation The expiry time of a key is unconditionally overwritten during instantiation, defaulting to turn it permanent. This causes a problem for DNS resolution as the expiration set by user-space is overwritten to TIME64_MAX, disabling further DNS updates. Fix this by restoring the condition that key_set_expiry is only called when the pre-parser sets a specific expiry.
CVE-2021-47548CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: ethernet: hisilicon: hns: hns_dsaf_misc: fix a possible array overflow in hns_dsaf_ge_srst_by_port() The if statement: if (port >= DSAF_GE_NUM) return; limits the value of port less than DSAF_GE_NUM (i.e., 8). However, if the value of port is 6 or 7, an array overflow could occur: port_rst_off = dsaf_dev->mac_cb[port]->port_rst_off; because the length of dsaf_dev->mac_cb is DSAF_MAX_PORT_NUM (i.e., 6). To fix this possible array overflow, we first check port and if it is greater than or equal to DSAF_MAX_PORT_NUM, the function returns.
CVE-2022-48716CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: ASoC: codecs: wcd938x: fix incorrect used of portid Mixer controls have the channel id in mixer->reg, which is not same as port id. port id should be derived from chan_info array. So fix this. Without this, its possible that we could corrupt struct wcd938x_sdw_priv by accessing port_map array out of range with channel id instead of port id.

References

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