• Dionna Glaze's avatar
    virt/coco/sev-guest: Add throttling awareness · 72f7754d
    Dionna Glaze authored
    A potentially malicious SEV guest can constantly hammer the hypervisor
    using this driver to send down requests and thus prevent or at least
    considerably hinder other guests from issuing requests to the secure
    processor which is a shared platform resource.
    
    Therefore, the host is permitted and encouraged to throttle such guest
    requests.
    
    Add the capability to handle the case when the hypervisor throttles
    excessive numbers of requests issued by the guest. Otherwise, the VM
    platform communication key will be disabled, preventing the guest from
    attesting itself.
    
    Realistically speaking, a well-behaved guest should not even care about
    throttling. During its lifetime, it would end up issuing a handful of
    requests which the hardware can easily handle.
    
    This is more to address the case of a malicious guest. Such guest should
    get throttled and if its VMPCK gets disabled, then that's its own
    wrongdoing and perhaps that guest even deserves it.
    
    To the implementation: the hypervisor signals with SNP_GUEST_REQ_ERR_BUSY
    that the guest requests should be throttled. That error code is returned
    in the upper 32-bit half of exitinfo2 and this is part of the GHCB spec
    v2.
    
    So the guest is given a throttling period of 1 minute in which it
    retries the request every 2 seconds. This is a good default but if it
    turns out to not pan out in practice, it can be tweaked later.
    
    For safety, since the encryption algorithm in GHCBv2 is AES_GCM, control
    must remain in the kernel to complete the request with the current
    sequence number. Returning without finishing the request allows the
    guest to make another request but with different message contents. This
    is IV reuse, and breaks cryptographic protections.
    
      [ bp:
        - Rewrite commit message and do a simplified version.
        - The stable tags are supposed to denote that a cleanup should go
          upfront before backporting this so that any future fixes to this
          can preserve the sanity of the backporter(s). ]
    
    Fixes: d5af44dd ("x86/sev: Provide support for SNP guest request NAEs")
    Signed-off-by: default avatarDionna Glaze <dionnaglaze@google.com>
    Co-developed-by: default avatarBorislav Petkov (AMD) <bp@alien8.de>
    Signed-off-by: default avatarBorislav Petkov (AMD) <bp@alien8.de>
    Reviewed-by: default avatarTom Lendacky <thomas.lendacky@amd.com>
    Cc: <stable@kernel.org> # d6fd48ef ("virt/coco/sev-guest: Check SEV_SNP attribute at probe time")
    Cc: <stable@kernel.org> # 970ab823 (" virt/coco/sev-guest: Simplify extended guest request handling")
    Cc: <stable@kernel.org> # c5a33827 ("virt/coco/sev-guest: Remove the disable_vmpck label in handle_guest_request()")
    Cc: <stable@kernel.org> # 0fdb6cc7 ("virt/coco/sev-guest: Carve out the request issuing logic into a helper")
    Cc: <stable@kernel.org> # d25bae7d ("virt/coco/sev-guest: Do some code style cleanups")
    Cc: <stable@kernel.org> # fa4ae42c ("virt/coco/sev-guest: Convert the sw_exit_info_2 checking to a switch-case")
    Link: https://lore.kernel.org/r/20230214164638.1189804-2-dionnaglaze@google.com
    72f7754d
sev-common.h 4.88 KB