Re: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and Indirect Branch Tracking for vsyscall emulation

[Andy Lutomirski]

On Fri, Sep 25, 2020 at 7:58 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
>
> Vsyscall entry points are effectively branch targets.  Mark them with
> ENDBR64 opcodes.  When emulating the RET instruction, unwind shadow stack
> and reset IBT state machine.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> ---
> v13:
> - Check shadow stack address is canonical.
> - Change from writing to MSRs to writing to CET xstate.
>
>  arch/x86/entry/vsyscall/vsyscall_64.c     | 34 +++++++++++++++++++++++
>  arch/x86/entry/vsyscall/vsyscall_emu_64.S |  9 ++++++
>  arch/x86/entry/vsyscall/vsyscall_trace.h  |  1 +
>  3 files changed, 44 insertions(+)
>
> diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
> index 44c33103a955..315ee3572664 100644
> --- a/arch/x86/entry/vsyscall/vsyscall_64.c
> +++ b/arch/x86/entry/vsyscall/vsyscall_64.c
> @@ -38,6 +38,9 @@
>  #include <asm/fixmap.h>
>  #include <asm/traps.h>
>  #include <asm/paravirt.h>
> +#include <asm/fpu/xstate.h>
> +#include <asm/fpu/types.h>
> +#include <asm/fpu/internal.h>
>
>  #define CREATE_TRACE_POINTS
>  #include "vsyscall_trace.h"
> @@ -286,6 +289,37 @@ bool emulate_vsyscall(unsigned long error_code,
>         /* Emulate a ret instruction. */
>         regs->ip = caller;
>         regs->sp += 8;
> +
> +#ifdef CONFIG_X86_CET
> +       if (tsk->thread.cet.shstk_size || tsk->thread.cet.ibt_enabled) {
> +               struct cet_user_state *cet;
> +               struct fpu *fpu;
> +
> +               fpu = &tsk->thread.fpu;
> +               fpregs_lock();
> +
> +               if (!test_thread_flag(TIF_NEED_FPU_LOAD)) {
> +                       copy_fpregs_to_fpstate(fpu);
> +                       set_thread_flag(TIF_NEED_FPU_LOAD);
> +               }
> +
> +               cet = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
> +               if (!cet) {
> +                       fpregs_unlock();
> +                       goto sigsegv;

I *think* your patchset tries to keep cet.shstk_size and
cet.ibt_enabled in sync with the MSR, in which case it should be
impossible to get here, but a comment and a warning would be much
better than a random sigsegv.

Shouldn't we have a get_xsave_addr_or_allocate() that will never
return NULL but instead will mark the state as in use and set up the
init state if the feature was previously not in use?