The Linux rethunk and safe-ret speculative safety techniques involve
transforming `ret` to `jmp __x86_return_thunk` at compile time. Placing naked
`ret`s back in executable .text breaks these mitigations.
CET-IBT requires ENDBR instructions, and while we could in principle fix that,
the need to select between ENDBR32 or ENDBR64 means that the contents of the
hypercall page would need to become more mode-specific than it currently
is (HVM hypercall pages are currently 32bit and 64bit compatbile). However,
there's no feasible way to make a hypercall page compatible with fine-grain
CFI schemes such as FineIBT.
OSes which care about either of these things are better off avoiding the
hypercall page.
This is part of XSA-466.
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Xen doesn't care (and indeed, cannot feasibly tell) whether a hypercall was
initiated using the hypercall page or not.
For SEV-SNP/TDX encrypted VMs, use of a hypercall page would violate the
integrity properties wanted.
Explicitly state that the hypercall page is optional.
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
The x86 hypercall ABI really used to have 6-argument hypercalls. V4V, the
downstream predecessor to Argo did take 6th args.
However, the 6th arg being %ebp in the 32bit ABI makes it unusable in
practice, because that's the frame pointer in builds with frame pointers
enabled. Therefore Argo was altered to being a 5-arg hypercall when it was
upstreamed.
c/s 2f531c122e ("x86: limit number of hypercall parameters to 5") removed
the ability for hypercalls to take 6 arguments.
Update the documentation to match reality.
Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>
Creative Commons is a more common license than GPL for documentation purposes.
Switch to using CC-BY-4.0 to explicitly permit re-purposing and remixing of
the content.
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Lars Kurth <lars.kurth@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
Sphinx, its linters, and RST modes in common editors, expect 3 spaces of
indentation. Some bits already conform to this expectation. Update the
rest to match.
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Lars Kurth <lars.kurth@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
Bernhard Kaindl
Andrew Cooper
Michal Orzel