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

Edgecombe, Rick P
2025-10-07T18:07:33Z

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