high CVSS 7.8

CVE-2021-47646·Kernel vulnerability

In the Linux kernel, the following vulnerability has been resolved: Revert "Revert "block, bfq: honor already-setup queue merges"" A crash [1] happened to be triggered in conjunction with commit 2d52c58b9c9b ("block, bfq: honor already-setup queue merges"). The latter was then reverted by commit ebc69e897e17 ("Revert "block, bfq: honor already-setup queue merges""). Yet, the reverted commit was not the one introducing the bug. In fact, it actually triggered a UAF introduced by a different commit, and now fixed by commit d29bd41428cf ("block, bfq: reset last_bfqq_created on group change"). So, there is no point in keeping commit 2d52c58b9c9b ("block, bfq: honor already-setup queue merges") out. This commit restores it. [1] https://bugzilla.kernel.org/show_bug.cgi?id=214503

Severity
high
Software
Kernel
Fixed in
5.17.2
Published
2025-02-26

Affected versions

From: 5.17

Until: 5.17.2

Fixed in: 5.17.2

How to fix this CVE

Update your Linux kernel to version 5.17.2 or later to resolve this use-after-free (UAF) vulnerability in the block I/O scheduler. The fix restores proper queue merge handling that was inadvertently reverted, eliminating kernel crashes that occur during block device operations. Apply this patch immediately to prevent potential system instability and data corruption.

sudo dnf update kernel kernel-devel

Defensia detects this vulnerability

How to check if you are affected

  1. Run `uname -r` to check your current kernel version; vulnerable versions are 5.17 through 5.17.1
  2. Check if BFQ I/O scheduler is enabled: `cat /sys/block/sda/queue/scheduler | grep bfq`
  3. Search kernel logs for UAF patterns: `sudo dmesg | grep -i "use-after-free\|UAF\|NULL pointer dereference"
  4. Verify the fix by confirming kernel version is 5.17.2 or later with `uname -r`

FAQ

What is CVE-2021-47646?

This CVE is a use-after-free vulnerability in the Linux kernel's BFQ block I/O scheduler that causes kernel crashes when queue merges are processed incorrectly, potentially leading to data corruption or denial of service.

Is CVE-2021-47646 being actively exploited?

No, this vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog and no public exploits are available.

What versions of Kernel are affected by CVE-2021-47646?

Linux kernel versions 5.17 through 5.17.1 are vulnerable; version 5.17.2 and later contain the fix.

How do I check if my server is vulnerable to CVE-2021-47646?

Run `uname -r` and compare your kernel version against 5.17.2; if your version is 5.17.0 or 5.17.1, you are vulnerable.

Does Defensia detect CVE-2021-47646?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Kernel is installed on a monitored server, CVE-2021-47646 will appear in your dashboard with remediation steps.

Related Kernel CVEs

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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-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-2024-38623CVSS 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-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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