Re: [PATCH v10 12/15] x86/traps: Communicate a LASS violation in #GP message

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

On Mon, 2025-10-06 at 23:51 -0700, Sohil Mehta wrote:
> From: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> 
> A LASS violation typically results in a #GP. Provide a more helpful
> message for a #GP when a kernel-side LASS violation is detected.
> 
> Currently, a kernel NULL pointer dereference triggers a #PF, which
> prints a helpful message. Because LASS enforcement is pre-paging,
> accesses to the first page frame would now be reported as a #GP, with an
> LASS violation hint. Add a special case to print a friendly message
> specifically for kernel NULL pointer dereferences.
> 
> Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
> ---
> v10:
>  - Minor improvement to code comments and hints.
> ---
>  arch/x86/kernel/traps.c | 45 ++++++++++++++++++++++++++++++-----------
>  1 file changed, 33 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 59bfbdf0a1a0..a5d10f7ae038 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -636,13 +636,23 @@ 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]	= "probably LASS violation for address",
> +	[GP_NULL_POINTER]	= "kernel NULL pointer dereference",
>  };
>  
>  /*
>   * When an uncaught #GP occurs, try to determine the memory address accessed by
>   * the instruction and return that address to the caller. Also, try to figure
> - * out whether any part of the access to that address was non-canonical.
> + * out whether any part of the access to that address was non-canonical or
> + * across privilege levels.
>   */
>  static enum kernel_gp_hint get_kernel_gp_address(struct pt_regs *regs,
>  						 unsigned long *addr)
> @@ -664,14 +674,27 @@ static enum kernel_gp_hint get_kernel_gp_address(struct pt_regs *regs,
>  		return GP_NO_HINT;

The patch looks good. I'm not sure how fancy we want to get here, but an idea if
you want it...

Above this hunk is:

	if (copy_from_kernel_nofault(insn_buf, (void *)regs->ip,
			MAX_INSN_SIZE))
		return EXC_NO_HINT;

If cpu_feature_enabled(X86_FEATURE_LASS) and regs->ip is in the lower half, we
could make a pretty strong guess that it was a LASS violation. I guess the same
argument could be made for the canonical guess work, but maybe calling a NULL
function pointer is more likely.

>  
>  #ifdef CONFIG_X86_64
> -	/*
> -	 * 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 active, a NULL pointer dereference generates a #GP
> +	 * instead of a #PF.
> +	 */
> +	if (*addr < PAGE_SIZE)
> +		return GP_NULL_POINTER;
> +
> +	/*
> +	 * Assume that LASS caused the exception, because the address is
> +	 * canonical and in the user half.
> +	 */
> +	if (cpu_feature_enabled(X86_FEATURE_LASS))
> +		return GP_LASS_VIOLATION;
>  #endif
>  
>  	return GP_CANONICAL;
> @@ -835,9 +858,7 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)
>  
>  	if (hint != GP_NO_HINT)
>  		snprintf(desc, sizeof(desc), GPFSTR ", %s 0x%lx",
> -			 (hint == GP_NON_CANONICAL) ? "probably for non-canonical address"
> -						    : "maybe for address",
> -			 gp_addr);
> +			 kernel_gp_hint_help[hint], gp_addr);
>  
>  	/*
>  	 * KASAN is interested only in the non-canonical case, clear it