On 2021-02-08 14:32, Will Deacon wrote:

Hi Marc,

On Mon, Feb 08, 2021 at 09:57:09AM +0000, Marc Zyngier wrote:

quoted

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.

Nailed it. Of course, before anything starts writing from C code, we 
need
to have initialised KASAN. kasan_init.c itself is compiled without any
address sanitising, but we can't repaint all the stuff that is called
from early_fdt_map() (quite a lot).

So the natural thing to do is to keep kasan_early_init() as the first
thing we do in C code, and everything falls from that.

Any chance you could try that on top and see if that cures your problem?
If that works for you, I'll push an updates series.

Thanks,

         M.

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index bce66d6bda74..09a5b603c950 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S

@@ -429,13 +429,13 @@ SYM_FUNC_START_LOCAL(__primary_switched)
  	bl	__pi_memset
  	dsb	ishst				// Make zero page visible to PTW

+#if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
+	bl	kasan_early_init
+#endif
  	mov	x0, x21				// pass FDT address in x0
  	bl	early_fdt_map			// Try mapping the FDT early
  	bl	init_feature_override
  	bl	switch_to_vhe
-#if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
-	bl	kasan_early_init
-#endif
  #ifdef CONFIG_RANDOMIZE_BASE
  	tst	x23, ~(MIN_KIMG_ALIGN - 1)	// already running randomized?
  	b.ne	0f

-- 
Jazz is not dead. It just smells funny...

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