Re: [PATCH] arm64: efi: Fix KASAN false positive for EFI runtime stack

[Will Deacon <will@kernel.org>]

On Fri, Jul 04, 2025 at 01:36:40PM +0100, Breno Leitao wrote:
> On Fri, Jul 04, 2025 at 10:26:37AM +0200, Ard Biesheuvel wrote:
> > On Thu, 3 Jul 2025 at 18:35, Catalin Marinas <catalin.marinas@arm.com> wrote:
> > > On Tue, Jun 24, 2025 at 05:55:53AM -0700, Breno Leitao wrote:
> ...
> > > >  arch/arm64/kernel/efi.c | 9 ++++++---
> ...
> > > >  static bool region_is_misaligned(const efi_memory_desc_t *md)
> > > >  {
> > > > @@ -214,9 +215,11 @@ static int __init arm64_efi_rt_init(void)
> > > >       if (!efi_enabled(EFI_RUNTIME_SERVICES))
> > > >               return 0;
> > > >
> > > > -     p = __vmalloc_node(THREAD_SIZE, THREAD_ALIGN, GFP_KERNEL,
> > > > -                        NUMA_NO_NODE, &&l);
> > > > -l:   if (!p) {
> > > > +     if (!IS_ENABLED(CONFIG_VMAP_STACK))
> > > > +             return -ENOMEM;
> > >
> > > Mark Rutland pointed out in a private chat that this should probably
> > > clear the EFI_RUNTIME_SERVICES flag as well.
> > >
> > 
> > If VMAP_STACK is a hard requirement, should we make CONFIG_EFI depend
> > on it for arm64?
> 
> What about if we make CONFIG_EFI select VMAP_STACK? I think it is more
> straight forward from a configuration perspective.
> 
> I thought about the following. What do you think?
> 
> 	arm64: EFI selects VMAP_STACK
> 
> 	Modify the ARM64 Kconfig to make the CONFIG_EFI configuration option
> 	automatically select CONFIG_VMAP_STACK.
> 
> 	The motivation is that arm64_efi_rt_init() will fail at runtime if
> 	CONFIG_VMAP_STACK is not set, so the patch ensures that enabling EFI
> 	will always enable VMAP_STACK as well, and avoid having EFI disabled in
> 	case the user didn't set VMAP_STACK.
> 
> 	Suggested-by: Ard Biesheuvel <ardb@kernel.org>
> 	Signed-off-by: Breno Leitao <leitao@debian.org>
> 
> 	diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> 	index 55fc331af3371..cc2585143f511 100644
> 	--- a/arch/arm64/Kconfig
> 	+++ b/arch/arm64/Kconfig
> 	@@ -2437,6 +2437,7 @@ config EFI
> 		select EFI_RUNTIME_WRAPPERS
> 		select EFI_STUB
> 		select EFI_GENERIC_STUB
> 	+	select VMAP_STACK
> 		imply IMA_SECURE_AND_OR_TRUSTED_BOOT
> 		default y
> 		help

I would actually like to select VMAP_STACK unconditionally for arm64.
Historically, we were held back waiting for all the various KASAN modes
to support vmalloc properly, but I _think_ that's fixed now...

The VMAP_STACK dependency is:

	depends on !KASAN || KASAN_HW_TAGS || KASAN_VMALLOC

and in arm64 we have:

	select KASAN_VMALLOC if KASAN

so it should be fine to select it afaict.

Any reason not to do that?

Will