Re: [PATCHv8 12/17] x86/traps: Communicate a LASS violation in #GP message

Sohil Mehta
2025-07-02T00:36:13Z

On 7/1/2025 2:58 AM, Kirill A. Shutemov wrote:
> /*
> @@ -672,6 +681,12 @@ static enum kernel_gp_hint get_kernel_gp_address(struct pt_regs *regs,
> if (*addr < ~__VIRTUAL_MASK &&
> *addr + insn.opnd_bytes - 1 > __VIRTUAL_MASK)
> return GP_NON_CANONICAL;
> + else if (*addr < ~__VIRTUAL_MASK &&
> + cpu_feature_enabled(X86_FEATURE_LASS)) {
> + if (*addr < PAGE_SIZE)
> + return GP_NULL_POINTER;
> + return GP_LASS_VIOLATION;
> + }

The comments above this section of code say:

/*
* Check that:
* - the operand is not in the kernel half
* - the last byte of the operand is not in the user canonical half
*/

They should be updated since we are updating the logic.

Also, below is easier to read than above:

if (*addr < ~__VIRTUAL_MASK) {

if (*addr + insn.opnd_bytes - 1 > __VIRTUAL_MASK)
return EXC_NON_CANONICAL;

if (cpu_feature_enabled(X86_FEATURE_LASS)) {
if (*addr < PAGE_SIZE)
return EXC_NULL_POINTER;
return EXC_LASS_VIOLATION;
}
}

I am wondering if the NULL pointer exception should be made
unconditional, even if it is unlikely to reach here without LASS. So
maybe something like this:

if (*addr < ~__VIRTUAL_MASK) {

if (*addr + insn.opnd_bytes - 1 > __VIRTUAL_MASK)
return EXC_NON_CANONICAL;

if (*addr < PAGE_SIZE)
return EXC_NULL_POINTER;

if (cpu_feature_enabled(X86_FEATURE_LASS))
return EXC_LASS_VIOLATION;
}