x86/sev: Add SEV-SNP guest feature negotiation support

The hypervisor can enable various new features (SEV_FEATURES[1:63]) and start a
SNP guest. Some of these features need guest side implementation. If any of
these features are enabled without it, the behavior of the SNP guest will be
undefined.  It may fail booting in a non-obvious way making it difficult to
debug.

Instead of allowing the guest to continue and have it fail randomly later,
detect this early and fail gracefully.

The SEV_STATUS MSR indicates features which the hypervisor has enabled.  While
booting, SNP guests should ascertain that all the enabled features have guest
side implementation. In case a feature is not implemented in the guest, the
guest terminates booting with GHCB protocol Non-Automatic Exit(NAE) termination
request event, see "SEV-ES Guest-Hypervisor Communication Block Standardization"
document (currently at https://developer.amd.com/wp-content/resources/56421.pdf),
section "Termination Request".

Populate SW_EXITINFO2 with mask of unsupported features that the hypervisor can
easily report to the user.

More details in the AMD64 APM Vol 2, Section "SEV_STATUS MSR".

  [ bp:
    - Massage.
    - Move snp_check_features() call to C code.
    Note: the CC:stable@ aspect here is to be able to protect older, stable
    kernels when running on newer hypervisors. Or not "running" but fail
    reliably and in a well-defined manner instead of randomly. ]

Fixes: cbd3d4f7c4e5 ("x86/sev: Check SEV-SNP features support")
Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Cc: <stable@kernel.org>
Link: https://lore.kernel.org/r/20230118061943.534309-1-nikunj@amd.com

1 files changed, 36 insertions, 0 deletions

diff --git a/Documentation/x86/amd-memory-encryption.rst b/Documentation/x86/amd-memory-encryption.rst
index a1940ebe7be5..934310ce7258 100644
--- a/Documentation/x86/amd-memory-encryption.rst
+++ b/Documentation/x86/amd-memory-encryption.rst
@@ -95,3 +95,39 @@ by supplying mem_encrypt=on on the kernel command line.  However, if BIOS does
 not enable SME, then Linux will not be able to activate memory encryption, even
 if configured to do so by default or the mem_encrypt=on command line parameter
 is specified.
+
+Secure Nested Paging (SNP)
+==========================
+
+SEV-SNP introduces new features (SEV_FEATURES[1:63]) which can be enabled
+by the hypervisor for security enhancements. Some of these features need
+guest side implementation to function correctly. The below table lists the
+expected guest behavior with various possible scenarios of guest/hypervisor
+SNP feature support.
+
++-----------------+---------------+---------------+------------------+
+| Feature Enabled | Guest needs   | Guest has     | Guest boot       |
+| by the HV       | implementation| implementation| behaviour        |
++=================+===============+===============+==================+
+|      No         |      No       |      No       |     Boot         |
+|                 |               |               |                  |
++-----------------+---------------+---------------+------------------+
+|      No         |      Yes      |      No       |     Boot         |
+|                 |               |               |                  |
++-----------------+---------------+---------------+------------------+
+|      No         |      Yes      |      Yes      |     Boot         |
+|                 |               |               |                  |
++-----------------+---------------+---------------+------------------+
+|      Yes        |      No       |      No       | Boot with        |
+|                 |               |               | feature enabled  |
++-----------------+---------------+---------------+------------------+
+|      Yes        |      Yes      |      No       | Graceful boot    |
+|                 |               |               | failure          |
++-----------------+---------------+---------------+------------------+
+|      Yes        |      Yes      |      Yes      | Boot with        |
+|                 |               |               | feature enabled  |
++-----------------+---------------+---------------+------------------+
+
+More details in AMD64 APM[1] Vol 2: 15.34.10 SEV_STATUS MSR
+
+[1] https://www.amd.com/system/files/TechDocs/40332.pdf