Re: [PATCHv9 11/16] x86/traps: Communicate a LASS violation in #GP message

[Sohil Mehta <sohil.mehta@intel.com>]

On 7/7/2025 1:03 AM, Kirill A. Shutemov wrote:
> From: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> 
> Provide a more helpful message on #GP when a kernel side LASS violation
> is detected.
> 
> A NULL pointer dereference is reported if a LASS violation occurs due to
> accessing the first page frame.
> 
> Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>  arch/x86/kernel/traps.c | 41 +++++++++++++++++++++++++++++------------
>  1 file changed, 29 insertions(+), 12 deletions(-)
> 

Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>

A nit below.

> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 59bfbdf0a1a0..4a4194e1d119 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -636,7 +636,16 @@ DEFINE_IDTENTRY(exc_bounds)
>  enum kernel_gp_hint {
>  	GP_NO_HINT,
>  	GP_NON_CANONICAL,
> -	GP_CANONICAL
> +	GP_CANONICAL,
> +	GP_LASS_VIOLATION,
> +	GP_NULL_POINTER,
> +};
> +
> +static const char * const kernel_gp_hint_help[] = {
> +	[GP_NON_CANONICAL]	= "probably for non-canonical address",
> +	[GP_CANONICAL]		= "maybe for address",
> +	[GP_LASS_VIOLATION]	= "LASS prevented access to address",
> +	[GP_NULL_POINTER]	= "kernel NULL pointer dereference",
>  };
>  
>  /*
> @@ -664,14 +673,23 @@ static enum kernel_gp_hint get_kernel_gp_address(struct pt_regs *regs,
>  		return GP_NO_HINT;
>  
>  #ifdef CONFIG_X86_64

Might as well get rid of the #ifdef in C code, if possible.

if (!IS_ENABLED(CONFIG_X86_64)
	return GP_CANONICAL;

or combine it with the next check.

> -	/*
> -	 * Check that:
> -	 *  - the operand is not in the kernel half
> -	 *  - the last byte of the operand is not in the user canonical half
> -	 */
> -	if (*addr < ~__VIRTUAL_MASK &&
> -	    *addr + insn.opnd_bytes - 1 > __VIRTUAL_MASK)
> +	/* Operand is in the kernel half */
> +	if (*addr >= ~__VIRTUAL_MASK)
> +		return GP_CANONICAL;
> +
> +	/* The last byte of the operand is not in the user canonical half */
> +	if (*addr + insn.opnd_bytes - 1 > __VIRTUAL_MASK)
>  		return GP_NON_CANONICAL;
> +
> +	/*
> +	 * If LASS is enabled, NULL pointer dereference generates
> +	 * #GP instead of #PF.
> +	 */
> +	if (*addr < PAGE_SIZE)
> +		return GP_NULL_POINTER;
> +
> +	if (cpu_feature_enabled(X86_FEATURE_LASS))
> +		return GP_LASS_VIOLATION;
>  #endif
>  
>  	return GP_CANONICAL;