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The HASHKEYR register contains a secret per-process key to enable unique hashes per process. In general it should not be exposed to userspace at all and a regular process has no need to know its key. However, checkpoint restore in userspace (CRIU) functionality requires that a process be able to set the HASHKEYR of another process, otherwise existing hashes on the stack would be invalidated by a new random key. Exposing HASHKEYR in this way also makes it appear in core dumps, which is a security concern. Multiple threads may share a key, for example just after a fork() call, where the kernel cannot know if the child is going to return back along the parent's stack. If such a thread is coerced into making a core dump, then the HASHKEYR value will be readable and able to be used against all other threads sharing that key, effectively undoing any protection offered by hashst/hashchk. Therefore we expose HASHKEYR to ptrace when CONFIG_CHECKPOINT_RESTORE is enabled, providing a choice of increased security or migratable ROP protected processes. This is similar to how ARM exposes its PAC keys. Signed-off-by: Benjamin Gray <bgray@linux.ibm.com> Reviewed-by: Russell Currey <ruscur@russell.cc> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> Link: https://msgid.link/20230616034846.311705-8-bgray@linux.ibm.com
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.