linux-kernel - [PATCH 0/6] objtool: Detect and warn about indirect calls in _

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Message-ID: <20250414111140.586315004@infradead.org>
Date: Mon, 14 Apr 2025 13:11:40 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: x86@...nel.org
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Subject: [PATCH 0/6] objtool: Detect and warn about indirect calls in __nocfi functions

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 existins [2] non-cfi indirect calls in the kernel.

Notably the KVM fastop emulation stuff -- which reminded me I still had pending
patches there. Included here since they reduce the amount of fastop call sites,
and the final patch includes an annotation for that. Although ideally we should
look at means of doing fastops differently.

KVM has another; the 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).

[1] https://lkml.kernel.org/r/20250410154556.GB9003@noisy.programming.kicks-ass.net
[2] https://lkml.kernel.org/r/20250410194334.GA3248459@google.com