Re: [PATCH] x86/tdx: Handle load_unaligned_zeropad() page-cross to a shared page

[Dave Hansen]

On 5/17/22 08:30, Kirill A. Shutemov wrote:
> load_unaligned_zeropad() can lead to unwanted loads across page boundaries.
> The unwanted loads are typically harmless. But, they might be made to
> totally unrelated or even unmapped memory. load_unaligned_zeropad()
> relies on exception fixup (#PF, #GP and now #VE) to recover from these
> unwanted loads.
> 
> In TDX guest the second page can be shared page and VMM may configure it
> to trigger #VE.
> 
> Kernel assumes that #VE on a shared page is MMIO access and tries to
> decode instruction to handle it. In case of load_unaligned_zeropad() it
> may result in confusion as it is not MMIO access.
> 
> Check fixup table before trying to handle MMIO.

Is this a theoretical problem or was it found in practice?

> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index 03deb4d6920d..5fbdda2f2b86 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -11,6 +11,8 @@
>  #include <asm/insn.h>
>  #include <asm/insn-eval.h>
>  #include <asm/pgtable.h>
> +#include <asm/trapnr.h>
> +#include <asm/extable.h>
>  
>  /* TDX module Call Leaf IDs */
>  #define TDX_GET_INFO			1
> @@ -296,6 +298,26 @@ static bool handle_mmio(struct pt_regs *regs, struct ve_info *ve)
>  	if (WARN_ON_ONCE(user_mode(regs)))
>  		return false;
>  
> +	/*
> +	 * load_unaligned_zeropad() relies on exception fixups in case of the
> +	 * word being a page-crosser and the second page is not accessible.
> +	 *
> +	 * In TDX guest the second page can be shared page and VMM may

	In TDX guests,

> +	 * configure it to trigger #VE.
> +	 *
> +	 * Kernel assumes that #VE on a shared page is MMIO access and tries to
> +	 * decode instruction to handle it. In case of load_unaligned_zeropad()
> +	 * it may result in confusion as it is not MMIO access.
> +	 *
> +	 * Check fixup table before trying to handle MMIO.
> +	 */
> +	if (fixup_exception(regs, X86_TRAP_VE, 0, ve->gla)) {
> +		/* regs->ip is adjusted by fixup_exception() */
> +		ve->instr_len = 0;
> +
> +		return true;
> +	}

This 've->instr_len = ' stuff is just a hack.

ve_info is a software structure.  Why not just add a:

	bool ip_adjusted;

which defaults to false, then we have:

	/*
	 * Adjust RIP if the exception was handled
	 * but RIP was not adjusted.
	 */
	if (!ret && !ve_info->ip_adjusted)
		regs->ip += ve_info->instr_len;

One other oddity I just stumbled upon:

static bool handle_mmio(struct pt_regs *regs, struct ve_info *ve)
{
...
        ve->instr_len = insn.length;

Why does that need to override 've->instr_len'?  What was wrong with the
gunk in r10 that came out of TDX_GET_VEINFO?