[PATCH v2 00/13] objtool: Detect and warn about indirect calls in __nocfi functions

Peter Zijlstra
2025-04-30T11:26:47Z

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@noisy.programming.kicks-ass.net
[2] https://lkml.kernel.org/r/20250410194334.GA3248459@google.com