Re: [PATCH 1/9] x86/cet: Control protection exception handler

Andy Lutomirski
2018-06-07T15:47:02Z

On Thu, Jun 7, 2018 at 7:40 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>
> A control protection exception is triggered when a control flow transfer
> attempt violated shadow stack or indirect branch tracking constraints.
> For example, the return address for a RET instruction differs from the
> safe copy on the shadow stack; or a JMP instruction arrives at a non-
> ENDBR instruction.
>
> The control protection exception handler works in a similar way as the
> general protection fault handler.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> ---
> arch/x86/entry/entry_32.S | 5 ++++
> arch/x86/entry/entry_64.S | 2 +-
> arch/x86/include/asm/traps.h | 3 +++
> arch/x86/kernel/idt.c | 1 +
> arch/x86/kernel/traps.c | 61 ++++++++++++++++++++++++++++++++++++++++++++
> 5 files changed, 71 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index bef8e2b202a8..14b63ef0d7d8 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -1070,6 +1070,11 @@ ENTRY(general_protection)
> jmp common_exception
> END(general_protection)
>
> +ENTRY(control_protection)
> + pushl $do_control_protection
> + jmp common_exception
> +END(control_protection)

Ugh, you're seriously supporting this on 32-bit? Please test double
fault handling very carefully -- the CET interaction with task
switches is so gross that I didn't even bother reading the spec except
to let the architects know that they were a but nuts to support it at
all.

> +
> #ifdef CONFIG_KVM_GUEST
> ENTRY(async_page_fault)
> ASM_CLAC
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 3166b9674429..5230f705d229 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -999,7 +999,7 @@ idtentry spurious_interrupt_bug do_spurious_interrupt_bug has_error_code=0
> idtentry coprocessor_error do_coprocessor_error has_error_code=0
> idtentry alignment_check do_alignment_check has_error_code=1
> idtentry simd_coprocessor_error do_simd_coprocessor_error has_error_code=0
> -
> +idtentry control_protection do_control_protection has_error_code=1
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 03f3d7695dac..4e8769a19aaf 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c

> +/*
> + * When a control protection exception occurs, send a signal
> + * to the responsible application. Currently, control
> + * protection is only enabled for the user mode. This
> + * exception should not come from the kernel mode.
> + */
> +dotraplinkage void
> +do_control_protection(struct pt_regs *regs, long error_code)
> +{
> + struct task_struct *tsk;
> +
> + RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
> + cond_local_irq_enable(regs);
> +
> + tsk = current;
> + if (!cpu_feature_enabled(X86_FEATURE_SHSTK) &&
> + !cpu_feature_enabled(X86_FEATURE_IBT)) {

static_cpu_has(), please. But your handling here is odd -- I think
that we should at least warn if we get #CP with CET disable.

> + goto exit;
> + }
> +
> + if (!user_mode(regs)) {
> + tsk->thread.error_code = error_code;
> + tsk->thread.trap_nr = X86_TRAP_CP;

I realize you copied this from elsewhere in the file, but please
either delete these assignments to error_code and trap_nr or at least
hoist them out of the if block.

> + if (notify_die(DIE_TRAP, "control protection fault", regs,
> + error_code, X86_TRAP_CP, SIGSEGV) != NOTIFY_STOP)

Does this notify_die() check serve any purpose at all? Removing all
the old ones would be a project, but let's try not to add new callers.

> + die("control protection fault", regs, error_code);
> + return;
> + }
> +
> + tsk->thread.error_code = error_code;
> + tsk->thread.trap_nr = X86_TRAP_CP;
> +
> + if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV) &&
> + printk_ratelimit()) {
> + unsigned int max_idx, err_idx;
> +
> + max_idx = ARRAY_SIZE(control_protection_err) - 1;
> + err_idx = min((unsigned int)error_code - 1, max_idx);

What if error_code == 0? Is that also invalid?

> + pr_info("%s[%d] control protection ip:%lx sp:%lx error:%lx(%s)",
> + tsk->comm, task_pid_nr(tsk),
> + regs->ip, regs->sp, error_code,
> + control_protection_err[err_idx]);
> + print_vma_addr(" in ", regs->ip);
> + pr_cont("\n");
> + }
> +
> +exit:
> + force_sig_info(SIGSEGV, SEND_SIG_PRIV, tsk);

This is definitely wrong for the feature-disabled, !user_mode case.

Also, are you planning on enabling CET for kernel code too?