# CVE-2021-4440: linux: In the Linux kernel, the following vulnerability has been resolved:  x86/xen: Drop USERGS_SYSRET64 paravirt call  com...

CVE-2021-4440 is fixed in linux on 1 platform by DEBIAN-CVE-2021-4440 (version 5.10.218-1), released June 25, 2024. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2021-4440
- Priority: High
- CVSS: 8.8 (high)
- EPSS: 0.0023
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jun 25, 2024

## 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   ]

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 11.6 (bullseye) | 5.10.218-1 | [DEBIAN-CVE-2021-4440](https://patchblox.com/security/data/fix/linux/debian-cve-2021-4440.md) | Jun 25, 2024 |

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