KVM: SEV: Return INVALID_INPUT on SNP req/resp buffer access failure

Currently, snp_handle_(ext_)guest_req() returns -EIO when
kvm_{read/clear}_guest() fails while accessing guest-provided buffers.
Returning -EIO causes KVM_RUN to exit to userspace, likely killing the VM.

Fix this by returning GHCB_HV_RESP_MALFORMED_INPUT with sub-error code
GHCB_ERR_INVALID_INPUT to the guest and resuming the vCPU.

Per the GHCB specification, guest-provided GPA buffers that cannot
be accessed by the hypervisor (e.g. private pages) should be treated
as guest input errors. Because kvm_{read/write/clear}_guest() only
returns -EFAULT on failure, treating this failure as an invalid input
aligns with the definition of -EFAULT ("Bad address").

Returning GHCB_ERR_INVALID_INPUT also matches existing SNP handling
in KVM, which already returns this error code for unaligned or
overlapping buffers. It also aligns with other hypercall implementations
in KVM (e.g. Hyper-V returning INVALID_HYPERCALL_INPUT on
kvm_read_guest() failures in kvm_hv_flush_tlb()).

Additionally, tickle the response buffer before issuing the PSP command
so that an invalid resp_gpa fails upfront with GHCB_ERR_INVALID_INPUT, i.e.
to avoid terminating the guest if it provides a bad buffer.  Do not return
an error to the guest if the post-firmware write fails, as the firmware has
already incremented the VMPCK sequence number.  Exiting to userspace avoids
creating an ambiguous failure where the guest has no way to know whether to
advance its VMPCK sequence number.  I.e. the "tickle" is a best effort
compromise between terminating the guest on bad input, and horribly
confusing the guest on bad *host userspace* behavior.

Fixes: 88caf544c930 ("KVM: SEV: Provide support for SNP_GUEST_REQUEST NAE event")
Fixes: 74458e4859d8 ("KVM: SEV: Provide support for SNP_EXTENDED_GUEST_REQUEST NAE event")
Signed-off-by: Jacky Li <jackyli@google.com>
Link: https://patch.msgid.link/20260923-snp-invalid-input-v2-1-c00e0dd077ca@google.com
[sean: massage comments and changelog regarding the tickle]
Signed-off-by: Sean Christopherson <seanjc@google.com>

0 files changed, 0 insertions, 0 deletions