Re: [PATCH V3 04/49] x86/xen: Add xenpv_restore_regs_and_return_to_usermode()

[Boris Ostrovsky]

On 10/13/21 11:13 PM, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
>
> While in the native case, PER_CPU_VAR(cpu_tss_rw + TSS_sp0) is the
> trampoline stack.  But XEN pv doesn't use trampoline stack, so
> PER_CPU_VAR(cpu_tss_rw + TSS_sp0) is also the kernel stack.  Hence source
> and destination stacks are identical in that case, which means reusing
> swapgs_restore_regs_and_return_to_usermode() in XEN pv would cause %rsp
> to move up to the top of the kernel stack and leave the IRET frame below
> %rsp, which is dangerous to be corrupted if #NMI / #MC hit as either of
> these events occurring in the middle of the stack pushing would clobber
> data on the (original) stack.
>
> And swapgs_restore_regs_and_return_to_usermode() pushing the IRET frame
> on to the original address is useless and error-prone when there is any
> future attempt to modify the code.
>
> Fixes: 7f2590a110b8 ("x86/entry/64: Use a per-CPU trampoline stack for IDT entries")
> Cc: Jan Beulich <jbeulich@xxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Juergen Gross <jgross@xxxxxxxx>
> Cc: Peter Anvin <hpa@xxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx>
> Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>


Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>