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SubjectRe: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and Indirect Branch Tracking for vsyscall emulation
On Fri, Sep 25, 2020 at 7:58 AM Yu-cheng Yu <yu-cheng.yu@intel.com> 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@intel.com>
> ---
> 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?

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