Re: [Xen-devel] [PATCH 5/7] xen/arm: traps: Avoid BUG_ON() in do_trap_brk()

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 23 Jul 2019, Julien Grall wrote:
> At the moment, do_trap_brk() is using a BUG_ON() to check the hardware
> has been correctly configured during boot.
> 
> Any error when configuring the hardware could result to a guest 'brk'
> trapping in the hypervisor and crash it.
> 
> This is pretty harsh to kill Xen when actually killing the guest would
> be enough as misconfiguring this trap would not lead to exposing
> sensitive data. Replace the BUG_ON() with crashing the guest.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> ---
>  xen/arch/arm/traps.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
> index 132686ee0f..ef37ca6bde 100644
> --- a/xen/arch/arm/traps.c
> +++ b/xen/arch/arm/traps.c
> @@ -1304,10 +1304,15 @@ int do_bug_frame(const struct cpu_user_regs *regs, 
> vaddr_t pc)
>  #ifdef CONFIG_ARM_64
>  static void do_trap_brk(struct cpu_user_regs *regs, const union hsr hsr)
>  {
> -    /* HCR_EL2.TGE and MDCR_EL2.TDE are not set so we never receive
> -     * software breakpoint exception for EL1 and EL0 here.
> +    /*
> +     * HCR_EL2.TGE and MDCR_EL2.TDR are currently not set. So we should
> +     * never receive software breakpoing exception for EL1 and EL0 here.
>       */
> -    BUG_ON(!hyp_mode(regs));
> +    if ( !hyp_mode(regs) )
> +    {
> +        domain_crash(current->domain);
> +        return;
> +    }

This is a good change to have. I am wondering if it would make sense to
also printk some debug messages, maybe even show_execution_state. If so,
we need to be careful that it's only done in debug builds, or it is rate
limited. What do you think? In any case, it is not necessary for this
patch to be accepted so:

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>

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