shim is a trivial EFI application that, when run, attempts to open and
execute another application. It will initially attempt to do this via the
standard EFI LoadImage() and StartImage() calls. If these fail (because Secure
Boot is enabled and the binary is not signed with an appropriate key, for
instance) it will then validate the binary against a built-in certificate. If
this succeeds and if the binary or signing key are not forbidden then shim
will relocate and execute the binary.
shim will also install a protocol which permits the second-stage bootloader to perform similar binary validation. This protocol has a GUID as described in the shim.h header file and provides a single entry point. On 64-bit systems this entry point expects to be called with SysV ABI rather than MSABI, so calls to it should not be wrapped.
Since version 16.1 shim overrides the system table and installs its own version of the LoadImage()/StartImage()/UnloadImage()/Exit() functions, so that second stages can simply call them from the system table, and it will work whether shim is first stage or not, without requiring shim-specific code in the second stages.
When this protocol is installed, signed UKIs Unified Kernel Images can be loaded even if the nested kernel is not signed, as after the UKI is loaded and validated, shim builds an internal allowlist of all the sections that are contained in the UKI. When an image is loaded from one such section, it is validated against denylists (DBX/MOKX/SBAT at the time of writing), but it is not checked against allowlists (DB/MOK hashes/signatures), as the outer image was already validated and the inner image is thus covered by those signatures or hashes. Furthermore, the inner image is not measured in the TPM, to avoid double measurements.
On systems with a TPM chip enabled and supported by the system firmware, shim will extend various PCRs with the digests of the targets it is loading. A full list is in the file README.tpm .
To use shim, simply place a DER-encoded public certificate in a file such as
pub.cer and build with make VENDOR_CERT_FILE=pub.cer.
There are a couple of build options, and a couple of ways to customize the build, described in BUILDING.
See the test plan, and file a ticket if anything fails!
In the event that the developers need to be contacted related to a security incident or vulnerability, please mail [email protected].
