Re: [Xen-devel] [PATCH v2 14/15] xen/arm: traps: Don't inject a fault if the translation VA -> IPA fails

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 23 May 2016, Julien Grall wrote:
> Based on ARM ARM (D4.5.3 in ARM DDI 0486A and B3.12.7 in ARM DDI 0406C.c),
> a Stage 1 translation error has priority over a Stage 2 translation error.
> 
> Therefore gva_to_ipa can only fail if another vCPU is playing with the
> page table.
> 
> Rather than injecting a custom fault, replay the instruction and let the
> processor injecting the correct fault.
> 
> This is fine as Xen is handling all the pending softirqs
> (see leave_hypervisor_tail) before returning to the guest. One of them
> is the scheduler which could reschuled the vCPU.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

Acked-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


>     Changes in v2:
>         - Update commit message to explain why a guest cannot DoS the
>         hypervisor with it.
> ---
>  xen/arch/arm/traps.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
> index c0325d5..3acdba0 100644
> --- a/xen/arch/arm/traps.c
> +++ b/xen/arch/arm/traps.c
> @@ -2410,7 +2410,7 @@ static void do_trap_instr_abort_guest(struct 
> cpu_user_regs *regs,
>  
>              rc = gva_to_ipa(gva, &gpa, GV2M_READ);
>              if ( rc == -EFAULT )
> -                goto bad_insn_abort;
> +                return; /* Try again */
>          }
>  
>          rc = p2m_mem_access_check(gpa, gva, npfec);
> @@ -2422,7 +2422,6 @@ static void do_trap_instr_abort_guest(struct 
> cpu_user_regs *regs,
>      break;
>      }
>  
> -bad_insn_abort:
>      inject_iabt_exception(regs, gva, hsr.len);
>  }
>  
> @@ -2452,7 +2451,7 @@ static void do_trap_data_abort_guest(struct 
> cpu_user_regs *regs,
>      {
>          rc = gva_to_ipa(info.gva, &info.gpa, GV2M_READ);
>          if ( rc == -EFAULT )
> -            goto bad_data_abort;
> +            return; /* Try again */
>      }
>  
>      switch ( dabt.dfsc & 0x3f )
> -- 
> 1.9.1
> 

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