Re: [PATCH v7 00/23] arm64: Early CPU feature override, and applications to VHE, BTI and PAuth

Will Deacon
2021-02-08T14:48:04Z

Hi Marc,

On Mon, Feb 08, 2021 at 09:57:09AM +0000, Marc Zyngier wrote:
> It recently came to light that there is a need to be able to override
> some CPU features very early on, before the kernel is fully up and
> running. The reasons for this range from specific feature support
> (such as using Protected KVM on VHE HW, which is the main motivation
> for this work) to errata workaround (a feature is broken on a CPU and
> needs to be turned off, or rather not enabled).
>
> This series tries to offer a limited framework for this kind of
> problems, by allowing a set of options to be passed on the
> command-line and altering the feature set that the cpufeature
> subsystem exposes to the rest of the kernel. Note that this doesn't
> change anything for code that directly uses the CPU ID registers.

I applied this locally, but I'm seeing consistent boot failure under QEMU when
KASAN is enabled. I tried sprinkling some __no_sanitize_address annotations
around (see below) but it didn't help. The culprit appears to be
early_fdt_map(), but looking a bit more closely, I'm really nervous about the
way we call into C functions from __primary_switched. Remember -- this code
runs _twice_ when KASLR is active: before and after the randomization. This
also means that any memory writes the first time around can be lost due to
the D-cache invalidation when (re-)creating the kernel page-tables.

Will

--->8

diff --git a/arch/arm64/kernel/idreg-override.c b/arch/arm64/kernel/idreg-override.c
index dffb16682330..751ed55261b5 100644
--- a/arch/arm64/kernel/idreg-override.c
+++ b/arch/arm64/kernel/idreg-override.c
@@ -195,7 +195,7 @@ static __init void parse_cmdline(void)
/* Keep checkers quiet */
void init_feature_override(void);

-asmlinkage void __init init_feature_override(void)
+asmlinkage void __init __no_sanitize_address init_feature_override(void)
{
int i;

diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
index 61845c0821d9..33581de05d2e 100644
--- a/arch/arm64/kernel/setup.c
+++ b/arch/arm64/kernel/setup.c
@@ -170,12 +170,12 @@ static void __init smp_build_mpidr_hash(void)

static void *early_fdt_ptr __initdata;

-void __init *get_early_fdt_ptr(void)
+void __init __no_sanitize_address *get_early_fdt_ptr(void)
{
return early_fdt_ptr;
}

-asmlinkage void __init early_fdt_map(u64 dt_phys)
+asmlinkage void __init __no_sanitize_address early_fdt_map(u64 dt_phys)
{
int fdt_size;