Re: [patch V5 01/38] x86/kvm/svm: Use uninstrumented wrmsrl() to restore GS

Jürgen Groß
2020-05-13T07:11:46Z

On 12.05.20 23:01, Thomas Gleixner wrote:
> On guest exit MSR_GS_BASE contains whatever the guest wrote to it and the
> first action after returning from the ASM code is to set it to the host
> kernel value. This uses wrmsrl() which is interesting at least.
>
> wrmsrl() is either using native_write_msr() or the paravirt variant. The
> XEN_PV code is uninteresting as nested SVM in a XEN_PV guest does not work.
>
> But native_write_msr() can be placed out of line by the compiler especially
> when paravirtualization is enabled in the kernel configuration. The
> function is marked notrace, but still can be probed if
> CONFIG_KPROBE_EVENTS_ON_NOTRACE is enabled.
>
> That would be a fatal problem as kprobe events use per-CPU variables which
> are GS based and would be accessed with the guest GS. Depending on the GS
> value this would either explode in colorful ways or lead to completely
> undebugable data corruption.
>
> Aside of that native_write_msr() contains a tracepoint which objtool
> complains about as it is invoked from the noinstr section.
>
> As this cannot run inside a XEN_PV guest there is no point in using
> wrmsrl(). Use native_wrmsrl() instead which is just a plain native WRMSR
> without tracing or anything else attached.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: Juergen Gross <jgross@suse.com>

Acked-by: Juergen Gross <jgross@suse.com>

Juergen