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== Background == There is a class of side-channel attacks against SGX enclaves called "SGX Step"[1]. These attacks create lots of exceptions inside of enclaves. Basically, run an in-enclave instruction, cause an exception. Over and over. There is a concern that a VMM could attack a TDX guest in the same way by causing lots of #VE's. The TDX architecture includes new countermeasures for these attacks. It basically counts the number of exceptions and can send another *special* exception once the number of VMM-induced #VE's hits a critical threshold[2]. == Problem == But, these special exceptions are independent of any action that the guest takes. They can occur anywhere that the guest executes. This includes sensitive areas like the entry code. The (non-paranoid) #VE handler is incapable of handling exceptions in these areas. == Solution == Fortunately, the special exceptions can be disabled by the guest via write to NOTIFY_ENABLES TDCS field. NOTIFY_ENABLES is disabled by default, but might be enabled by a bootloader, firmware or an earlier kernel before the current kernel runs. Disable NOTIFY_ENABLES feature explicitly and unconditionally. Any NOTIFY_ENABLES-based #VE's that occur before this point will end up in the early #VE exception handler and die due to unexpected exit reason. [1] https://github.com/jovanbulck/sgx-step [2] https://intel.github.io/ccc-linux-guest-hardening-docs/security-spec.html#safety-against-ve-in-kernel-code Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Reviewed-by: Dave Hansen <dave.hansen@intel.com> Link: https://lore.kernel.org/all/20230126221159.8635-8-kirill.shutemov%40linux.intel.com |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.