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CVE-2021-4440

Description

In the Linux kernel, the following vulnerability has been resolved:x86/xen: Drop USERGS_SYSRET64 paravirt callcommit afd30525a659ac0ae0904f0cb4a2ca75522c3123 upstream.USERGS_SYSRET64 is used to return from a syscall via SYSRET, buta Xen PV guest will nevertheless use the IRET hypercall, as thereis no sysret PV hypercall defined.So instead of testing all the prerequisites for doing a sysret andthen mangling the stack for Xen PV again for doing an iret just usethe iret exit from the beginning.This can easily be done via an ALTERNATIVE like it is done for thesysenter compat case already.It should be noted that this drops the optimization in Xen for notrestoring a few registers when returning to user mode, but it seemsas if the saved instructions in the kernel more than compensate forthis drop (a kernel build in a Xen PV guest was slightly faster withthis patch applied).While at it remove the stale sysret32 remnants. [ pawan: Brad Spengler and Salvatore Bonaccorso 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 ]

POC

Reference

No PoCs from references.

Github

- https://github.com/AlexRogalskiy/AlexRogalskiy

- https://github.com/kherrick/hacker-news

- https://github.com/kherrick/lobsters

- https://github.com/pawan-shivarkar/pawan-shivarkar

- https://github.com/xairy/linux-kernel-exploitation