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    SubjectRe: [PATCH RFC v9 13/51] x86/fault: Handle RMP page faults for user addresses
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    On 6/11/23 21:25, Michael Roth wrote:
    > From: Brijesh Singh <brijesh.singh@amd.com>
    >
    > When SEV-SNP is enabled globally, a write from the host is subject to
    > checks performed by the hardware against the RMP table (APM2 15.36.10)
    > at the end of a page walk:
    >
    > 1. Assigned bit in the RMP table is not set (i.e page is shared).
    > 2. Immutable bit in the RMP table is not set.
    > 3. If the page table entry that gives the sPA indicates that the
    > target page size is a large page, then all RMP entries for the 4KB
    > constituting pages of the target must have the assigned bit 0.
    >
    > Nothing constructive can come of an attempt by userspace to violate case
    > 1) (which will result in writing garbage due to page encryption) or case
    > 2) (userspace should not ever need or be allowed to write to a page that
    > the host has specifically needed to mark immutable).

    What does this _mean_? If nothing constructive can come of it, what
    does that mean for the kernel?

    > Case 3) is dependent on the hypervisor. In case of KVM, due to how
    > shared/private pages are partitioned into separate memory pools via
    > restricted/guarded memory, there should never be a case where a page in
    > the private pool overlaps with a shared page: either it is a
    > hugepage-sized allocation and all the sub-pages are private, or it is a
    > single-page allocation, in which case it cannot overlap with anything
    > but itself.
    >
    > Therefore, for all 3 cases, it is appropriate to simply kill the
    > userspace process if it ever generates an RMP #PF. Implement that logic
    > here.
    ...
    > + if (error_code & X86_PF_RMP) {
    > + pr_err("Unexpected RMP page fault for address 0x%lx, terminating process\n",
    > + address);
    > + do_sigbus(regs, error_code, address, VM_FAULT_SIGBUS);
    > + return;
    > + }
    > +

    This is special-snowflake code. You're making the argument that an RMP
    fault is a special snowflake and needs special handling.

    Why should an RMP violation be any different than, say a write to a
    read-only page (that also ends in signal delivery)?

    I kinda dislike the entire changelog here. I really don't know what
    point it's making or what it is arguing.

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