CVE-2021-4440

Advisory lineage Upstream: 0 Downstream: 5
Analyzed
Published: 25 Jun 2024, 14:20
Last modified:11 May 2026, 13:43

Vulnerability Summary

Overall Risk (default)
medium
35/100
CVSS Score
8.8 HIGH
v3.1 (cve.org)
EPSS Score
0.03% LOW
0% probability 0.00%
KEV
Not listed
Ransomware
No reports
Public exploits
None found
Dark Web
Not detected

Timeline

25 Jun 2024, 14:20
Published
Vulnerability first disclosed
11 May 2026, 13:43
Last Modified
Vulnerability information updated

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 <carnil@debian.org> 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 ]

CVSS Metrics

  • v3.1•HIGH•Score: 8.8CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

EPSS Trends

Current EPSS score: 0.03%• Percentile: 10%

Techniques & Countermeasures

  • CWE-400•Uncontrolled Resource Consumption

    The product does not properly control the allocation and maintenance of a limited resource.

Affected Systems

  • linux•linux

    ≥ cea750c99d8f6391080c420f811a46b21bad7cf4, < 1424ab4bb386df9cc590c73afa55f13e9b00dea2 | ≥ 5.10.215, < 5.10.218

  • linux•linux_kernel

    ≥ 5.10.215, < 5.10.218

References (2)