Re: [PATCH v7 04/10] x86/tdx: Handle early IO operations

[Sean Christopherson]

On Tue, Oct 05, 2021, Kuppuswamy Sathyanarayanan wrote:
> Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>

Heh, is Andi double-dipping to pad his stats?  :-D

> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> ---

...

> diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c
> index 11e367228e96..4cbffcb737d9 100644
> --- a/arch/x86/kernel/tdx.c
> +++ b/arch/x86/kernel/tdx.c
> @@ -10,6 +10,11 @@
>  /* TDX Module call Leaf IDs */
>  #define TDGETVEINFO			3
>  
> +#define VE_IS_IO_OUT(exit_qual)		(((exit_qual) & 8) ? 0 : 1)
> +#define VE_GET_IO_SIZE(exit_qual)	(((exit_qual) & 7) + 1)
> +#define VE_GET_PORT_NUM(exit_qual)	((exit_qual) >> 16)
> +#define VE_IS_IO_STRING(exit_qual)	((exit_qual) & 16 ? 1 : 0)
> +
>  /*
>   * Allocate it in the data region to avoid zeroing it during
>   * BSS initialization. It is mainly used in cc_platform_has()
> @@ -228,6 +233,61 @@ int tdx_handle_virtualization_exception(struct pt_regs *regs,
>  	return ret;
>  }
>  
> +/*
> + * Handle early IO, mainly for early printks serial output.
> + * This avoids anything that doesn't work early on, like tracing
> + * or printks, by calling the low level functions directly. Any
> + * problems are handled by falling back to a standard early exception.
> + *
> + * Assumes the IO instruction was using ax, which is enforced
> + * by the standard io.h macros.
> + */
> +static __init bool tdx_early_io(struct pt_regs *regs, u32 exit_qual)
> +{
> +	struct tdx_hypercall_output outh;

"outh" looks like a typo.  Maybe "result" or something alongs those lines?

> +	int out, size, port, ret;
> +	bool string;
> +	u64 mask;
> +
> +	string = VE_IS_IO_STRING(exit_qual);
> +
> +	/* I/O strings ops are unrolled at build time. */
> +	if (string)

Why bother with "string"?

	if (VE_IS_IO_STRING(exit_qual))
		return false;

> +		return 0;

Ugh.  This needs to be "return false".  "return 0" in the kernel usually means
success, but this horror returns a bool where "false" is failure.

> +
> +	out = VE_IS_IO_OUT(exit_qual);
> +	size = VE_GET_IO_SIZE(exit_qual);
> +	port = VE_GET_PORT_NUM(exit_qual);
> +	mask = GENMASK(8 * size, 0);

size * BITS_PER_BYTE

> +
> +	ret = _tdx_hypercall(EXIT_REASON_IO_INSTRUCTION, size, out, port,
> +			     regs->ax, &outh);

This unnecessarily exposes RAX to the untrusted VMM for IN.

> +	if (!out && !ret) {
> +		regs->ax &= ~mask;
> +		regs->ax |= outh.r11 & mask;
> +	}
> +
> +	return !ret;
> +}