x86/xen: Drop USERGS_SYSRET64 paravirt call

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

]