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 ]
CVSS Details
- CVSS 3.1 Base Score: 8.8
- CVSS 3.1 Vector: (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H)
Covered by Rapid7
| Product | Vendor Advisory | Solution File | Added | Published |
|---|---|---|---|---|
| Amazon Linux Ami 2 | — | Upgrade kernel-toolsUpgrade kernel-tools-develUpgrade kernel-develUpgrade kernel-debuginfoUpgrade kernel-tools-debuginfoUpgrade perfUpgrade kernel-debuginfo-common-aarch64Upgrade bpftoolUpgrade kernel-debuginfo-common-x86_64Upgrade perf-debuginfoUpgrade python-perf-debuginfoUpgrade bpftool-debuginfoUpgrade python-perfUpgrade kernel-livepatch-5.10.218-206.860Upgrade kernelUpgrade kernel-headers | Aug 2, 2024 | Jun 25, 2024 |
| Debian | — | Upgrade linux | Jul 30, 2024 | Jun 25, 2024 |
| Redhat_linux | — | No solution exists | Jul 9, 2025 | Jun 25, 2024 |
| Ubuntu | — | No solution existsUpgrade linux-azure-fde-5.15 | Jun 26, 2025 | Jun 25, 2024 |
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