Re: [PATCH v6 48/76] x86/entry/64: Add entry code for #VC handler

Borislav Petkov
2020-08-31T11:31:03Z

On Mon, Aug 24, 2020 at 10:54:43AM +0200, Joerg Roedel wrote:
> @@ -446,6 +448,82 @@ _ASM_NOKPROBE(\asmsym)
> SYM_CODE_END(\asmsym)
> .endm
>

ifdeffery pls...

> +/**
> + * idtentry_vc - Macro to generate entry stub for #VC
> + * @vector: Vector number
> + * @asmsym: ASM symbol for the entry point
> + * @cfunc: C function to be called
> + *
> + * The macro emits code to set up the kernel context for #VC. The #VC handler
> + * runs on an IST stack and needs to be able to cause nested #VC exceptions.
> + *
> + * To make this work the #VC entry code tries its best to pretend it doesn't use
> + * an IST stack by switching to the task stack if coming from user-space (which
> + * includes early SYSCALL entry path) or back to the stack in the IRET frame if
> + * entered from kernel-mode.
> + *
> + * If entered from kernel-mode the return stack is validated first, and if it is
> + * not safe to use (e.g. because it points to the entry stack) the #VC handler
> + * will switch to a fall-back stack (VC2) and call a special handler function.
> + *
> + * The macro is only used for one vector, but it is planned to extend it in the
^^^^^^^^^^^

"... to be extended..."

...

> @@ -674,6 +675,56 @@ asmlinkage __visible noinstr struct pt_regs *sync_regs(struct pt_regs *eregs)
> return regs;
> }
>
> +#ifdef CONFIG_AMD_MEM_ENCRYPT
> +asmlinkage __visible noinstr struct pt_regs *vc_switch_off_ist(struct pt_regs *eregs)
> +{
> + unsigned long sp, *stack;
> + struct stack_info info;
> + struct pt_regs *regs;

Let's call those "regs_ret" or so, so that the argument can be "regs" by
convention and for better differentiation.

> + /*
> + * In the SYSCALL entry path the RSP value comes from user-space - don't
> + * trust it and switch to the current kernel stack
> + */
> + if (eregs->ip >= (unsigned long)entry_SYSCALL_64 &&
> + eregs->ip < (unsigned long)entry_SYSCALL_64_safe_stack) {
> + sp = this_cpu_read(cpu_current_top_of_stack);
> + goto sync;
> + }
> +
> + /*
> + * From here on the the RSP value is trusted - more RSP sanity checks
> + * need to happen above.
> + *
> + * Check whether entry happened from a safe stack.
> + */
> + sp = eregs->sp;
> + stack = (unsigned long *)sp;
> + get_stack_info_noinstr(stack, current, &info);
> +
> + /*
> + * Don't sync to entry stack or other unknown stacks - use the fall-back
> + * stack instead.
> + */
> + if (info.type == STACK_TYPE_UNKNOWN || info.type == STACK_TYPE_ENTRY ||

AFAICT, that STACK_TYPE_UNKNOWN gets set only by the plain
get_stack_info() function - not by the _noinstr() variant so you'd need
to check the return value of latter...

> + info.type >= STACK_TYPE_EXCEPTION_LAST)
> + sp = __this_cpu_ist_top_va(VC2);
> +
> +sync:
> + /*
> + * Found a safe stack - switch to it as if the entry didn't happen via
> + * IST stack. The code below only copies pt_regs, the real switch happens
> + * in assembly code.
> + */
> + sp = ALIGN_DOWN(sp, 8) - sizeof(*regs);
> +
> + regs = (struct pt_regs *)sp;
> + *regs = *eregs;
> +
> + return regs;
> +}
> +#endif
> +
> struct bad_iret_stack {
> void *error_entry_ret;
> struct pt_regs regs;
> --

--
Regards/Gruss,
Boris.