Re: [PATCH 4/4] arm64: kexec: Change relocate_kernel to C code

From: Mark Rutland <mark.rutland@arm.com>
To: Pingfan Liu <piliu@redhat.com>
Cc: linux-arm-kernel@lists.infradead.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Ard Biesheuvel <ardb@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Pasha Tatashin <pasha.tatashin@soleen.com>
Subject: Re: [PATCH 4/4] arm64: kexec: Change relocate_kernel to C code
Date: Tue, 2 Apr 2024 10:55:08 +0100	[thread overview]
Message-ID: <ZgvV_FR-5I6Pu8D-@FVFF77S0Q05N> (raw)
In-Reply-To: <20240328115656.24090-5-piliu@redhat.com>

On Thu, Mar 28, 2024 at 07:56:54PM +0800, Pingfan Liu wrote:
> The kexec_relocate.o is a self-contained section, and it should be PIE.
> 
> Beside that, C function call requires stack, which is built on the idmap
> of the rear of kimage->control_code_page.
> 
> Signed-off-by: Pingfan Liu <piliu@redhat.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
> To: linux-arm-kernel@lists.infradead.org
> ---
>  arch/arm64/kernel/Makefile          |   1 +
>  arch/arm64/kernel/asm-offsets.c     |  10 --
>  arch/arm64/kernel/machine_kexec.c   |   9 +-
>  arch/arm64/kernel/relocate_kernel.S | 100 --------------
>  arch/arm64/kernel/relocate_kernel.c | 197 ++++++++++++++++++++++++++++
>  arch/arm64/kernel/vmlinux.lds.S     |   1 +
>  6 files changed, 206 insertions(+), 112 deletions(-)
>  delete mode 100644 arch/arm64/kernel/relocate_kernel.S
>  create mode 100644 arch/arm64/kernel/relocate_kernel.c

> +static void  __kexec_section turn_mmu_off(void)
> +{
> +	u64 tmp = INIT_SCTLR_EL1_MMU_OFF;
> +
> +	/* pre_disable_mmu_workaround */
> +#ifdef CONFIG_QCOM_FALKOR_ERRATUM_E1041
> +	isb();
> +#endif
> +	write_sysreg(tmp, sctlr_el1);
> +	isb();
> +}

Disabling the MMU cannot be done from C; as soon as we write to SCTLR_EL1 (even
before the ISB) we cannot safely access the stack until that has been explcitly
cleaned+invalidated to the PoC (and that has to be done by VA).

I don't think we should bother trying to move this to C; the MMU-off portions
should remain as asssembly.

If you want to move the MMU-on portions to C, then *maybe* that's worthwhile, but 
given the diffstat I reckon it's better to leave this all as asm for now. We
can make this more legibile without converting it to C.

Mark.

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