Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support

Vlastimil Babka
2021-11-23T08:55:11Z

On 11/22/21 23:51, Dave Hansen wrote:
> On 11/22/21 12:33 PM, Brijesh Singh wrote:
>>> How do we, for example, prevent ptrace() from inducing a panic()?
>>
>> In the current approach, this access will induce a panic().
>
> That needs to get fixed before SEV-SNP is merged, IMNHO. This behavior
> would effectively mean that any userspace given access to create SNP
> guests would panic the kernel.
>
>> In general, supporting the ptrace() for the encrypted VM region is
>> going to be difficult.
>
> By "supporting", do you mean doing something functional? I don't really
> care if ptrace() to guest private memory returns -EINVAL or whatever.
> The most important thing is not crashing the host.
>
> Also, as Sean mentioned, this isn't really about ptrace() itself. It's
> really about ensuring that no kernel or devices accesses to guest
> private memory can induce bad behavior.

Then we need gup to block any changes from shared to guest private? I assume
there will be the usual issues of recognizing temporary elevated refcount vs
long-term gup, etc.

>> The upcoming TDX work to unmap the guest memory region from the
>> current process page table can easily extend for the SNP to cover the
>> current limitations.

By "current process page table" you mean userspace page tables?

> My preference would be that we never have SEV-SNP code in the kernel
> that can panic() the host from guest userspace. If that means waiting
> until there's common guest unmapping infrastructure around, then I think
> we should wait.
>