Re: [PATCH v2 1/2] x86/sev-es: Allow copy_from_kernel_nofault in earlier boot

[Dave Hansen <dave.hansen@intel.com>]

On 9/20/23 13:37, Sean Christopherson wrote:
> On Mon, Sep 11, 2023, Adam Dunlap wrote:
>> Previously, if copy_from_kernel_nofault was called before
>> boot_cpu_data.x86_virt_bits was set up, then it would trigger undefined
>> behavior due to a shift by 64. This ended up causing boot failures in
>> the latest version of ubuntu2204 in the gcp project when using SEV-SNP.
>> Specifically, this function is called during an early #VC handler which
>> is triggered by a cpuid to check if nx is implemented.
> Why not stuff boot_cpu_data.x86_virt_bits to a "default" value that is guaranteed
> to be accurate (or at least safe) for the purposes of the early boot code.  I.e.
> set it to 48 for 64-bit kernels.
> 
> That'd avoid the extra conditional, and would avoid laying whack-a-mole with
> anything else that consumes x86_virt_bits.

I'd be worried that could break things even more subtly.

If we're truly worried about whack-a-mole, we should stick
'x86_virt_bits' in a wrapper, whine if it's accessed inadvertently, and
*then* return some mostly sane data.

That way we can actually go look at the caller and see what the heck
it's doing.

I did poke around and managed to convince myself that this site _is_ the
only one at the moment.