Hi! On kCFI (CONFIG_CFI_CLANG=y) builds all indirect calls should have the CFI check on (with very few exceptions). Not having the CFI checks undermines the protection provided by CFI and will make these sites candidates for people wanting to steal your cookies. Specifically the ABI changes are so that doing indirect calls without the CFI magic, to a CFI adorned function is not compatible (although it happens to work for some setups, it very much does not for FineIBT). Rust people tripped over this the other day, since their 'core' happened to have some no_sanitize(kcfi) bits in, which promptly exploded when ran with FineIBT on. Since this is very much not a supported model -- on purpose, have objtool detect and warn about such constructs. This effort [1] found all existing [2] non-cfi indirect calls in the kernel. Notably the KVM fastop emulation stuff -- which I've completely rewritten for this version -- the generated code doesn't look horrific, but is slightly more verbose. I'm running on the assumption that instruction emulation is not super performance critical these days of zero VM-exit VMs etc. KVM has another; the VMX interrupt injection stuff calls the IDT handler directly. Is there an alternative? Can we keep a table of Linux functions slighly higher up the call stack (asm_\cfunc ?) and add CFI to those? HyperV hypercall page stuff, which I've previously suggested use direct calls, and which I've now converted (after getting properly annoyed with that code). Also available at: git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git x86/core Changes since v1: - complete rewrite of the fastop stuff - HyperV tweaks (Michael) - objtool changes (Josh) [1] https://lkml.kernel.org/r/20250410154556.GB9003@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx [2] https://lkml.kernel.org/r/20250410194334.GA3248459@xxxxxxxxxx
