CVE-2021-4440 x86/xen: Drop USERGS_SYSRET64 paravirt call

High 8.8 Published 25 Jun 2024 Updated 11 May 2026

A missing CPU buffer clearing in Xen PV guests can lead to denial of service or information leakage, requiring kernel patching.

Exploited in the wild: Not known
Login needed: Any signed-in user
Attackable: Local or same network only
Victim action needed: Not needed
Fix: Available

What is CVE-2021-4440?

CVE-2021-4440 is a high-severity denial of service vulnerability in Linux kernel, published on 25 Jun 2024. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

In 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 ]

Weakness: CWE-400

Which versions are affected by CVE-2021-4440?

It affects 1 product.

Products include Linux kernel.

How do I fix CVE-2021-4440?

Apply the vendor's patch.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2021-4440 being exploited?

There is no known exploitation of CVE-2021-4440 in the wild and no public exploit code.

All Linux kernel vulnerabilities

Sources

Also known as DEBIAN-CVE-2021-4440, GHSA-vc52-cgfp-jw27, UBUNTU-CVE-2021-4440.