high CVSS 7.8

CVE-2021-47111·Kernel vulnerability

In the Linux kernel, the following vulnerability has been resolved: xen-netback: take a reference to the RX task thread Do this in order to prevent the task from being freed if the thread returns (which can be triggered by the frontend) before the call to kthread_stop done as part of the backend tear down. Not taking the reference will lead to a use-after-free in that scenario. Such reference was taken before but dropped as part of the rework done in 2ac061ce97f4. Reintroduce the reference taking and add a comment this time explaining why it's needed. This is XSA-374 / CVE-2021-28691.

Severity
high
Software
Kernel
Fixed in
5.12.10
Published
2024-03-15

Affected versions

From: 5.11

Until: 5.12.10

Fixed in: 5.12.10

How to fix this CVE

Upgrade your Linux kernel to version 5.12.10 or later to patch a use-after-free vulnerability in the xen-netback driver that could allow local privilege escalation. The fix restores proper reference counting for the RX task thread to prevent premature task cleanup during backend teardown. Systems running Xen virtualization with kernel versions 5.11 through 5.12.9 should prioritize this update.

sudo dnf update kernel kernel-devel

Defensia detects this vulnerability

How to check if you are affected

  1. Step 1: Check current kernel version with `uname -r` and compare against affected range 5.11.x to 5.12.9
  2. Step 2: Verify xen-netback driver is in use by running `lsmod | grep xen_netback` (relevant only on Xen guest/host systems)
  3. Step 3: Search system logs for kernel panic or NULL pointer dereference messages: `grep -i 'use.*after.*free\|null.*pointer' /var/log/kern.log /var/log/dmesg`
  4. Step 4: After patching, confirm new kernel version with `uname -r` and reboot to activate the update

FAQ

What is CVE-2021-47111?

A use-after-free vulnerability in the Linux kernel's xen-netback networking driver where the RX task thread could be deallocated prematurely if the frontend triggers thread termination before the backend completes its teardown, potentially allowing local privilege escalation on Xen-virtualized systems.

Is CVE-2021-47111 being actively exploited?

No, this vulnerability is not listed in CISA's Known Exploited Vulnerabilities (KEV) catalog and no public exploits are available, though it remains a high-severity local privilege escalation risk.

What versions of Kernel are affected by CVE-2021-47111?

Linux kernel versions 5.11.0 through 5.12.9 are vulnerable; version 5.12.10 and later contain the fix.

How do I check if my server is vulnerable to CVE-2021-47111?

Run `uname -r` to get your kernel version; if it reports 5.11.x or 5.12.x (below 5.12.10) and you're using Xen virtualization, your system is vulnerable.

Does Defensia detect CVE-2021-47111?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Kernel is installed on a monitored server, CVE-2021-47111 will appear in your dashboard with remediation steps.

Related Kernel CVEs

CVE-2026-100075CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: RDMA/srpt: Fix srpt_alloc_rw_ctxs() unwind counters When srpt_alloc_rw_ctxs() fails partway through a multi-buffer indirect descriptor, the unwind path destroys RDMA contexts but leaves stale n_rw_ctx and n_rdma values (and a dangling rw_ctxs pointer). Later sq_wr_avail accounting in srpt_queue_response() or srpt_write_pending() can then subtract the wrong number of send queue credits. Reset the counters and clear rw_ctxs after freeing the heap allocation before returning an error.
CVE-2026-64035CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: igc: set tx buffer type for SMD frames Sashiko pointed out that igc_fpe_init_smd_frame() initializes igc_tx_buffer fields for an SMD skb, but does not set the buffer type: https://sashiko.dev/#/patchset/20260415025226.114115-1-kohei%40enjuk.jp Since igc_tx_buffer entries are reused, a stale XDP or XSK type can remain and make TX completion use the wrong cleanup path. Set the buffer type to IGC_TX_BUFFER_TYPE_SKB.
CVE-2026-64033CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: RDMA/rtrs: Fix use-after-free in path file creation cleanup In the error path of rtrs_srv_create_path_files(), the sysfs root folders may already have been created and srv_path->kobj may already have been initialized. If a later step fails, the cleanup currently calls kobject_put(&srv_path->kobj) before rtrs_srv_destroy_once_sysfs_root_folders(srv_path). kobject_put() may drop the last reference to srv_path->kobj and invoke the release callback, rtrs_srv_release(), which frees srv_path. The following call to rtrs_srv_destroy_once_sysfs_root_folders(srv_path) then dereferences srv_path internally to access srv_path->srv, resulting in a use-after-free. This failure path is reached before rtrs_srv_create_path_files() returns success, so the successful-path lifetime handling is not involved. Fix this by destroying the sysfs root folders before calling kobject_put(&srv_path->kobj), so srv_path is still valid while the helper accesses it. This issue was found by a static analysis tool I am developing.
CVE-2026-64037CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: wifi: iwlwifi: mld: fix TSO segmentation explosion when AMSDU is disabled When the TLC notification disables AMSDU for a TID, the MLD driver sets max_tid_amsdu_len to the sentinel value 1. The TSO segmentation path in iwl_mld_tx_tso_segment() checks for zero but not for this sentinel, allowing it to reach the num_subframes calculation: num_subframes = (max_tid_amsdu_len + pad) / (subf_len + pad) = (1 + 2) / (1534 + 2) = 0 This zero propagates to iwl_tx_tso_segment() which sets: gso_size = num_subframes * mss = 0 Calling skb_gso_segment() with gso_size=0 creates over 32000 tiny segments from a single GSO skb. This floods the TX ring with ~1024 micro-frames (the rest are purged), creating a massive burst of TX completion events that can lead to memory corruption and a subsequent use-after-free in TCP's retransmit queue (refcount underflow in tcp_shifted_skb, NULL deref in tcp_rack_detect_loss). The MVM driver is immune because it checks mvmsta->amsdu_enabled before reaching the num_subframes calculation. The MLD driver has no equivalent bitmap check and relies solely on max_tid_amsdu_len, which does not catch the sentinel value. Fix this by detecting the sentinel value (max_tid_amsdu_len == 1) at the existing check and falling back to non-AMSDU TSO segmentation. Also add a WARN_ON_ONCE guard after the num_subframes division as defense-in-depth to catch any future code paths that produce zero through a different mechanism.
CVE-2026-64000CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: net: hsr: fix potential OOB access in supervision frame handling Ensure the entire TLV header is linearized before access by adding sizeof(struct hsr_sup_tlv) to the pskb_may_pull() calls. Without this, a truncated frame could cause an out-of-bounds access.

References

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