Linux KernelOperating system · Linux
CVE-2021-4440
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedIn the Linux kernel, the following vulnerability has been resolved: x86/xen: Drop USERGS_SYSRET64 paravirt call commit afd30525a659ac0ae0904f0cb4a2ca75522c3123 upstream. USERGS_SYSRET64 is used to return from a syscall via SYSRET, but a Xen PV guest will nevertheless use the IRET hypercall, as there is no sysret PV hypercall defined. So instead of testing all the prerequisites for doing a sysret and then mangling the stack for Xen PV again for doing an iret just use the iret exit from the beginning. This can easily be done via an ALTERNATIVE like it is done for the sysenter compat case already. It should be noted that this drops the optimization in Xen for not restoring a few registers when returning to user mode, but it seems as if the saved instructions in the kernel more than compensate for this drop (a kernel build in a Xen PV guest was slightly faster with this patch applied). While at it remove the stale sysret32 remnants. [ pawan: Brad Spengler and Salvatore Bonaccorso <[email protected]> reported a problem with the 5.10 backport commit edc702b4a820 ("x86/entry_64: Add VERW just before userspace transition"). When CONFIG_PARAVIRT_XXL=y, CLEAR_CPU_BUFFERS is not executed in syscall_return_via_sysret path as USERGS_SYSRET64 is runtime patched to: .cpu_usergs_sysret64 = { 0x0f, 0x01, 0xf8, 0x48, 0x0f, 0x07 }, // swapgs; sysretq which is missing CLEAR_CPU_BUFFERS. It turns out dropping USERGS_SYSRET64 simplifies the code, allowing CLEAR_CPU_BUFFERS to be explicitly added to syscall_return_via_sysret path. Below is with CONFIG_PARAVIRT_XXL=y and this patch applied: syscall_return_via_sysret: ... <+342>: swapgs <+345>: xchg %ax,%ax <+347>: verw -0x1a2(%rip) <------ <+354>: sysretq ]
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysisA single request can consume unbounded CPU, memory, or connections, so a modest amount of malicious traffic exhausts the service. The result is denial of service for everyone else. Remediation is enforcing limits, quotas, and timeouts on what any one request can use.
General guidance for the uncontrolled resource consumption class — the official description and references above are authoritative for this specific CVE. Want a bespoke review and a reviewed fix? Ask our team →
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data
Linux KernelOperating system
>= 5.10.215, < 5.10.218CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Local
- Complexity
- Low
- Privileges
- Low
- User interaction
- None
- Scope
- Changed
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped5.10.218
Linux Kernel 5.10.218
- Upgrade the Linux kernel to version 5.10.218 or later. This version includes the fix from commit 1424ab4bb386df9cc590c73afa55f13e9b00dea2 which drops USERGS_SYSRET64 paravirt call and ensures CLEAR_CPU_BUFFERS (VERW instruction) is executed in the syscall_return_via_sysret path.
- Verify the current kernel version using `uname -r` or `cat /proc/version` to confirm the upgrade was successful.
- If compiling from source, ensure the kernel config includes the appropriate PARAVIRT settings; the fix works correctly with CONFIG_PARAVIRT_XXL=y.
Generated from the published advisory — verify against the referenced sources before acting.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-4440 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-4440 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Version or environment caveats, and links to real fixes
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