On 7/7/2025 1:03 AM, Kirill A. Shutemov wrote:
> From: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxx>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
> arch/x86/kernel/traps.c | 41 +++++++++++++++++++++++++++++------------
> 1 file changed, 29 insertions(+), 12 deletions(-)
>
Reviewed-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>
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;
