high CVSS 7.1

CVE-2021-47255·Kernel vulnerability

In the Linux kernel, the following vulnerability has been resolved: kvm: LAPIC: Restore guard to prevent illegal APIC register access Per the SDM, "any access that touches bytes 4 through 15 of an APIC register may cause undefined behavior and must not be executed." Worse, such an access in kvm_lapic_reg_read can result in a leak of kernel stack contents. Prior to commit 01402cf81051 ("kvm: LAPIC: write down valid APIC registers"), such an access was explicitly disallowed. Restore the guard that was removed in that commit.

Severity
high
Software
Kernel
Fixed in
5.12.13
Published
2024-05-21

Affected versions

From: 5.11

Until: 5.12.13

Fixed in: 5.12.13

How to fix this CVE

Update your Linux kernel to version 5.12.13 or later to address a memory disclosure vulnerability in the KVM LAPIC subsystem. This vulnerability allows unauthorized reads of kernel stack memory through malformed APIC register access patterns. Ensure all virtualization hosts are patched promptly, as this affects KVM-based hypervisors running vulnerable kernel versions.

sudo dnf check-update kernel && 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 the affected range (5.11.0 through 5.12.12)
  2. Step 2: Verify if KVM is enabled on the system by running `grep -c '^processor' /proc/cpuinfo` and checking if any VMs are running with `virsh list --all` (if libvirt is installed)
  3. Step 3: Search kernel logs for APIC-related errors using `sudo dmesg | grep -i 'apic\|lapic'` and `sudo journalctl -b | grep -i 'apic'`
  4. Step 4: After patching, confirm the new kernel is loaded with `uname -r` and verify it is version 5.12.13 or later

FAQ

What is CVE-2021-47255?

This vulnerability is a kernel memory disclosure flaw in KVM's Local APIC (LAPIC) register handling that permits guest virtual machines to read uninitialized kernel stack memory by issuing invalid APIC register access patterns. The issue was introduced when input validation checks were removed in a prior commit.

Is CVE-2021-47255 being actively exploited?

No, this vulnerability is not listed on the CISA Known Exploited Vulnerabilities (KEV) catalog and no public exploits are currently available. However, it remains a high-severity issue due to potential information disclosure risks.

What versions of Kernel are affected by CVE-2021-47255?

Linux kernel versions 5.11.0 through 5.12.12 are vulnerable. The fix was merged into stable releases starting with 5.12.13.

How do I check if my server is vulnerable to CVE-2021-47255?

Run `uname -r` and check if your kernel version falls within 5.11.0 to 5.12.12. If it does and KVM is enabled (check with `lsmod | grep kvm`), your system is vulnerable.

Does Defensia detect CVE-2021-47255?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Kernel is installed on a monitored server, CVE-2021-47255 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-2024-38612CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: ipv6: sr: fix invalid unregister error path The error path of seg6_init() is wrong in case CONFIG_IPV6_SEG6_LWTUNNEL is not defined. In that case if seg6_hmac_init() fails, the genl_unregister_family() isn't called. This issue exist since commit 46738b1317e1 ("ipv6: sr: add option to control lwtunnel support"), and commit 5559cea2d5aa ("ipv6: sr: fix possible use-after-free and null-ptr-deref") replaced unregister_pernet_subsys() with genl_unregister_family() in this error path.
CVE-2024-39462CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: clk: bcm: dvp: Assign ->num before accessing ->hws Commit f316cdff8d67 ("clk: Annotate struct clk_hw_onecell_data with __counted_by") annotated the hws member of 'struct clk_hw_onecell_data' with __counted_by, which informs the bounds sanitizer about the number of elements in hws, so that it can warn when hws is accessed out of bounds. As noted in that change, the __counted_by member must be initialized with the number of elements before the first array access happens, otherwise there will be a warning from each access prior to the initialization because the number of elements is zero. This occurs in clk_dvp_probe() due to ->num being assigned after ->hws has been accessed: UBSAN: array-index-out-of-bounds in drivers/clk/bcm/clk-bcm2711-dvp.c:59:2 index 0 is out of range for type 'struct clk_hw *[] __counted_by(num)' (aka 'struct clk_hw *[]') Move the ->num initialization to before the first access of ->hws, which clears up the warning.
CVE-2024-38541CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: of: module: add buffer overflow check in of_modalias() In of_modalias(), if the buffer happens to be too small even for the 1st snprintf() call, the len parameter will become negative and str parameter (if not NULL initially) will point beyond the buffer's end. Add the buffer overflow check after the 1st snprintf() call and fix such check after the strlen() call (accounting for the terminating NUL char).
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.

References

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