Re: [PATCH 1/2] x86/tdx: Fix off-by-one in port I/O handling

[Kuppuswamy Sathyanarayanan]

Hi Kirill,

On 3/31/2026 4:24 AM, Kiryl Shutsemau (Meta) wrote:
> handle_in() and handle_out() in arch/x86/coco/tdx/tdx.c use:
> 
>     u64 mask = GENMASK(BITS_PER_BYTE * size, 0);
> 
> GENMASK(h, l) includes bit h. For size=1 (INB), this produces
> GENMASK(8, 0) = 0x1FF (9 bits) instead of GENMASK(7, 0) = 0xFF (8
> bits). The mask is one bit too wide for all I/O sizes.
> 
> Fix the mask calculation.
> 
> Fixes: 03149948832a ("x86/tdx: Port I/O: Add runtime hypercalls")
> Reported-by: Borys Tsyrulnikov <tsyrulnikov.borys@xxxxxxxxx>
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---

LGTM. Can you include a link to the bug report or related discussion in 
the commit log? It will help understand the impact of this issue.

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

>  arch/x86/coco/tdx/tdx.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index 7b2833705d47..4d7f71d50122 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -693,7 +693,7 @@ static bool handle_in(struct pt_regs *regs, int size, int port)
>  		.r13 = PORT_READ,
>  		.r14 = port,
>  	};
> -	u64 mask = GENMASK(BITS_PER_BYTE * size, 0);
> +	u64 mask = GENMASK(BITS_PER_BYTE * size - 1, 0);
>  	bool success;
>  
>  	/*
> @@ -713,7 +713,7 @@ static bool handle_in(struct pt_regs *regs, int size, int port)
>  
>  static bool handle_out(struct pt_regs *regs, int size, int port)
>  {
> -	u64 mask = GENMASK(BITS_PER_BYTE * size, 0);
> +	u64 mask = GENMASK(BITS_PER_BYTE * size - 1, 0);
>  
>  	/*
>  	 * Emulate the I/O write via hypercall. More info about ABI can be found

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer