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Message-ID: <ZPj6gxSUc8SLzpLQ@gmail.com>
Date: Thu, 7 Sep 2023 00:17:39 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,
Borislav Petkov <bp@...en8.de>, x86@...nel.org,
Ashok Raj <ashok.raj@...el.com>,
Arjan van de Ven <arjan@...ux.intel.com>,
Fenghua Yu <fenghua.yu@...el.com>, Peter Anvin <hpa@...or.com>
Subject: Re: [patch 2/2] x86/boot/32: Disable stackprotector and tracing for
mk_early_pgtbl_32()
* Thomas Gleixner <tglx@...utronix.de> wrote:
> Stackprotector cannot work before paging is enabled. The read from the per
> CPU variable __stack_chk_guard is always accessing the virtual address
> either directly on UP or via FS on SMP. In physical address mode this
> results in an access to memory above 3GB.
>
> So this works by chance as the hardware returns the same value when there
> is no RAM at this physical address. When there is RAM populated above 3G
> then the read is by chance the same as nothing changes that memory during
> the very early boot stage.
>
> Stop relying on pure luck and disable the stack protector for the only C
> function which is called during early boot before paging is enabled.
>
> Remove function tracing from the whole source file as there is no way to
> trace this at all, but in case of CONFIG_DYNAMIC_FTRACE=n
> mk_early_pgtbl_32() would access global function tracer variables in
> physcial address mode which again might work by chance.
>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
> arch/x86/kernel/Makefile | 1 +
> arch/x86/kernel/head32.c | 3 ++-
> 2 files changed, 3 insertions(+), 1 deletion(-)
Reviewed-by: Ingo Molnar <mingo@...nel.org>
Thanks,
Ingo
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