Re: [PATCH 1/1] kbuild: Fix CFI hash randomization with KASAN

Kees Cook
2023-01-12T22:52:39Z

On Thu, Jan 12, 2023 at 10:49:48PM +0000, Sami Tolvanen wrote:
> Clang emits a asan.module_ctor constructor to each object file
> when KASAN is enabled, and these functions are indirectly called
> in do_ctors. With CONFIG_CFI_CLANG, the compiler also emits a CFI
> type hash before each address-taken global function so they can
> pass indirect call checks.
>
> However, in commit 0c3e806ec0f9 ("x86/cfi: Add boot time hash
> randomization"), x86 implemented boot time hash randomization,
> which relies on the .cfi_sites section generated by objtool. As
> objtool is run against vmlinux.o instead of individual object
> files with X86_KERNEL_IBT (enabled by default), CFI types in
> object files that are not part of vmlinux.o end up not being
> included in .cfi_sites, and thus won't get randomized and trip
> CFI when called.
>
> Only .vmlinux.export.o and init/version-timestamp.o are linked
> into vmlinux separately from vmlinux.o. As these files don't
> contain any functions, disable KASAN for both of them to avoid
> breaking hash randomization.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1742
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

--
Kees Cook