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Re: [PATCH] x86/kexec: Fix kexec-reboot with CET active


  • To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Wed, 30 Mar 2022 11:00:21 +0200
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  • Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, David Vrabel <dvrabel@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Wed, 30 Mar 2022 09:00:52 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 07.03.2022 21:53, Andrew Cooper wrote:
> --- a/xen/arch/x86/machine_kexec.c
> +++ b/xen/arch/x86/machine_kexec.c
> @@ -156,6 +156,16 @@ void machine_kexec(struct kexec_image *image)
>       */
>      local_irq_disable();
>  
> +    /* Reset CPUID masking and faulting to the host's default. */
> +    ctxt_switch_levelling(NULL);
> +
> +    /* Disable CET. */
> +    if ( read_cr4() & X86_CR4_CET )
> +    {
> +        wrmsrl(MSR_S_CET, 0);
> +        write_cr4(read_cr4() & ~X86_CR4_CET);
> +    }

It just occurred to me: Isn't using read_cr4() here somewhat risky?
If we crashed on the CR4 write in write_cr4(), we'd try to load CR4
here again with a value which may cause another fault.

Jan




 


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