From 93de2a6a4b91b72607136dd656edf03fb399d27f Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Wed, 23 Sep 2026 09:37:21 -0700 Subject: KVM: SEV: Do cache maintenance on the source VM during intra-host migration Manually perform cache maintenance on the source VM during intra-host migration to ensure no stale data is left in CPU caches after the VM is destroyed. Because the source VM is "converted" to a non-SEV VM, KVM's memory reclaim flows won't trigger cache maintenance, e.g. when all guest memory is reclaimed in response to detaching from the mmu_notifier. Note, relying on the destination VM to do cache maintenance isn't an option as KVM doesn't require identical guest memory configurations, i.e. the source VM may have access to memory that the destination VM does not. Enforcing equivalent memory configurations is infeasible, as it would require a *deep* comparison of memslots, e.g. to verify that not only are the memslot identical, but what the memslots point at is also identical. Fixes: b56639318bb2 ("KVM: SEV: Add support for SEV intra host migration") Cc: stable@vger.kernel.org Reported-by: Stefan Teodorescu Signed-off-by: Sean Christopherson Message-ID: <20260923163721.1584779-3-seanjc@google.com> Signed-off-by: Paolo Bonzini --- Documentation/admin-guide/module-signing.rst | 292 +++++++++++++++++++++++++++ 1 file changed, 292 insertions(+) create mode 100644 Documentation/admin-guide/module-signing.rst (limited to 'Documentation/admin-guide/module-signing.rst') diff --git a/Documentation/admin-guide/module-signing.rst b/Documentation/admin-guide/module-signing.rst new file mode 100644 index 000000000..7f2f127dc --- /dev/null +++ b/Documentation/admin-guide/module-signing.rst @@ -0,0 +1,292 @@ +Kernel module signing facility +------------------------------ + +.. CONTENTS +.. +.. - Overview. +.. - Configuring module signing. +.. - Generating signing keys. +.. - Public keys in the kernel. +.. - Manually signing modules. +.. - Signed modules and stripping. +.. - Loading signed modules. +.. - Non-valid signatures and unsigned modules. +.. - Administering/protecting the private key. + + +======== +Overview +======== + +The kernel module signing facility cryptographically signs modules during +installation and then checks the signature upon loading the module. This +allows increased kernel security by disallowing the loading of unsigned modules +or modules signed with an invalid key. Module signing increases security by +making it harder to load a malicious module into the kernel. The module +signature checking is done by the kernel so that it is not necessary to have +trusted userspace bits. + +This facility uses X.509 ITU-T standard certificates to encode the public keys +involved. The signatures are not themselves encoded in any industrial standard +type. The built-in facility currently only supports the RSA, NIST P-384 ECDSA +and NIST FIPS-204 ML-DSA public key signing standards (though it is pluggable +and permits others to be used). For RSA and ECDSA, the possible hash +algorithms that can be used are SHA-2 and SHA-3 of sizes 256, 384, and 512 (the +algorithm is selected by data in the signature); ML-DSA does its own hashing, +but is allowed to be used with a SHA512 hash for signed attributes. + + +========================== +Configuring module signing +========================== + +The module signing facility is enabled by going to the +:menuselection:`Enable Loadable Module Support` section of +the kernel configuration and turning on:: + + CONFIG_MODULE_SIG "Module signature verification" + +This has a number of options available: + + (1) :menuselection:`Require modules to be validly signed` + (``CONFIG_MODULE_SIG_FORCE``) + + This specifies how the kernel should deal with a module that has a + signature for which the key is not known or a module that is unsigned. + + If this is off (ie. "permissive"), then modules for which the key is not + available and modules that are unsigned are permitted, but the kernel will + be marked as being tainted, and the concerned modules will be marked as + tainted, shown with the character 'E'. + + If this is on (ie. "restrictive"), only modules that have a valid + signature that can be verified by a public key in the kernel's possession + will be loaded. All other modules will generate an error. + + Irrespective of the setting here, if the module has a signature block that + cannot be parsed, it will be rejected out of hand. + + + (2) :menuselection:`Automatically sign all modules` + (``CONFIG_MODULE_SIG_ALL``) + + If this is on then modules will be automatically signed during the + modules_install phase of a build. If this is off, then the modules must + be signed manually using:: + + scripts/sign-file + + + (3) :menuselection:`Which hash algorithm should modules be signed with?` + + This presents a choice of which hash algorithm the installation phase will + sign the modules with: + + =============================== ========================================== + ``CONFIG_MODULE_SIG_SHA256`` :menuselection:`Sign modules with SHA-256` + ``CONFIG_MODULE_SIG_SHA384`` :menuselection:`Sign modules with SHA-384` + ``CONFIG_MODULE_SIG_SHA512`` :menuselection:`Sign modules with SHA-512` + ``CONFIG_MODULE_SIG_SHA3_256`` :menuselection:`Sign modules with SHA3-256` + ``CONFIG_MODULE_SIG_SHA3_384`` :menuselection:`Sign modules with SHA3-384` + ``CONFIG_MODULE_SIG_SHA3_512`` :menuselection:`Sign modules with SHA3-512` + =============================== ========================================== + + The algorithm selected here will also be built into the kernel (rather + than being a module) so that modules signed with that algorithm can have + their signatures checked without causing a dependency loop. + + + (4) :menuselection:`File name or PKCS#11 URI of module signing key` + (``CONFIG_MODULE_SIG_KEY``) + + Setting this option to something other than its default of + ``certs/signing_key.pem`` will disable the autogeneration of signing keys + and allow the kernel modules to be signed with a key of your choosing. + The string provided should identify a file containing both a private key + and its corresponding X.509 certificate in PEM form, or — on systems where + the OpenSSL ENGINE_pkcs11 is functional — a PKCS#11 URI as defined by + RFC7512. In the latter case, the PKCS#11 URI should reference both a + certificate and a private key. + + If the PEM file containing the private key is encrypted, or if the + PKCS#11 token requires a PIN, this can be provided at build time by + means of the ``KBUILD_SIGN_PIN`` variable. + + + (5) :menuselection:`Additional X.509 keys for default system keyring` + (``CONFIG_SYSTEM_TRUSTED_KEYS``) + + This option can be set to the filename of a PEM-encoded file containing + additional certificates which will be included in the system keyring by + default. + +Note that enabling module signing adds a dependency on the OpenSSL devel +packages to the kernel build processes for the tool that does the signing. + + +======================= +Generating signing keys +======================= + +Cryptographic keypairs are required to generate and check signatures. A +private key is used to generate a signature and the corresponding public key is +used to check it. The private key is only needed during the build, after which +it can be deleted or stored securely. The public key gets built into the +kernel so that it can be used to check the signatures as the modules are +loaded. + +Under normal conditions, when ``CONFIG_MODULE_SIG_KEY`` is unchanged from its +default, the kernel build will automatically generate a new keypair using +openssl if one does not exist in the file:: + + certs/signing_key.pem + +during the building of vmlinux (the public part of the key needs to be built +into vmlinux) using parameters in the:: + + certs/x509.genkey + +file (which is also generated if it does not already exist). + +One can select between RSA (``MODULE_SIG_KEY_TYPE_RSA``), ECDSA +(``MODULE_SIG_KEY_TYPE_ECDSA``) and ML-DSA (``MODULE_SIG_KEY_TYPE_MLDSA_*``) to +generate an RSA 4k, a NIST P-384 keypair or an ML-DSA 44, 65 or 87 keypair. + +It is strongly recommended that you provide your own x509.genkey file. + +Most notably, in the x509.genkey file, the req_distinguished_name section +should be altered from the default:: + + [ req_distinguished_name ] + #O = Unspecified company + CN = Build time autogenerated kernel key + #emailAddress = unspecified.user@unspecified.company + +The generated RSA key size can also be set with:: + + [ req ] + default_bits = 4096 + + +It is also possible to manually generate the key private/public files using the +x509.genkey key generation configuration file in the root node of the Linux +kernel sources tree and the openssl command. The following is an example to +generate the public/private key files:: + + openssl req -new -nodes -utf8 -sha256 -days 36500 -batch -x509 \ + -config x509.genkey -outform PEM -out kernel_key.pem \ + -keyout kernel_key.pem + +The full pathname for the resulting kernel_key.pem file can then be specified +in the ``CONFIG_MODULE_SIG_KEY`` option, and the certificate and key therein will +be used instead of an autogenerated keypair. + + +========================= +Public keys in the kernel +========================= + +The kernel contains a ring of public keys that can be viewed by root. They're +in a keyring called ".builtin_trusted_keys" that can be seen by:: + + [root@deneb ~]# cat /proc/keys + ... + 223c7853 I------ 1 perm 1f030000 0 0 keyring .builtin_trusted_keys: 1 + 302d2d52 I------ 1 perm 1f010000 0 0 asymmetri Fedora kernel signing key: d69a84e6bce3d216b979e9505b3e3ef9a7118079: X509.RSA a7118079 [] + ... + +Beyond the public key generated specifically for module signing, additional +trusted certificates can be provided in a PEM-encoded file referenced by the +``CONFIG_SYSTEM_TRUSTED_KEYS`` configuration option. + +Further, the architecture code may take public keys from a hardware store and +add those in also (e.g. from the UEFI key database). + +Finally, it is possible to add additional public keys by doing:: + + keyctl padd asymmetric "" [.builtin_trusted_keys-ID] <[key-file] + +e.g.:: + + keyctl padd asymmetric "" 0x223c7853