Re: [Xen-devel] [PATCH for-4.13 v4 11/19] xen/arm: Ensure the SSBD workaround is re-enabled right after exiting a guest

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 31 Oct 2019, Julien Grall wrote:
> At the moment, SSBD workaround is re-enabled for Xen after interrupts
> are unmasked. This means we may end up to execute some part of the
> hypervisor if an interrupt is received before the workaround is
> re-enabled.
> 
> Each trap may require to unmask different interrupts.
> As the rest of enter_hypervisor_from_guest() does not require to have
> interrupts masked, the function is now split in two parts:
>     1) enter_hypervisor_from_guest_preirq() called with interrupts
>        masked.
>     2) enter_hypervisor_from_guest() called with interrupts unmasked.
> 
> Note that while it might be possible to avoid spliting the function in
> two parts, it requires a bit more work than I can currently invest to
> avoid using indirect branch.
> 
> Furthermore, the function name is rather generic as there might be more
> work to dob before interrupts are unmasked in the future.
> 
> Fixes: a7898e4c59 ("xen/arm: Add ARCH_WORKAROUND_2 support for guests")
> Reported-by: Andrii Anisov <andrii_anisov@xxxxxxxx>
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
>     Changes in v4:
>         - Remove spurious line
> 
>     Changes in v3:
>         - Rework the arm32 part
> 
>     Changes in v2:
>         - Add Arm32 code
>         - Rename enter_hypervisor_from_guest_noirq() to
>         enter_hypervisor_from_guest_preirq()
>         - Update the commit message to explain the choice of splitting
>         the code.
> ---
>  xen/arch/arm/arm32/entry.S |  2 +-
>  xen/arch/arm/arm64/entry.S |  1 +
>  xen/arch/arm/traps.c       | 14 ++++++++++++--
>  3 files changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/xen/arch/arm/arm32/entry.S b/xen/arch/arm/arm32/entry.S
> index cea4e0e302..0a9c248ee2 100644
> --- a/xen/arch/arm/arm32/entry.S
> +++ b/xen/arch/arm/arm32/entry.S
> @@ -118,7 +118,7 @@ abort_guest_exit_end:
>          bne return_from_trap
>  
>  skip_check:
> -        mov pc, lr
> +        b   enter_hypervisor_from_guest_preirq
>  ENDPROC(arch_enter_hypervisor_from_guest_preirq)
>  
>          /*
> diff --git a/xen/arch/arm/arm64/entry.S b/xen/arch/arm/arm64/entry.S
> index 97dc60210d..d4fb5fdc1c 100644
> --- a/xen/arch/arm/arm64/entry.S
> +++ b/xen/arch/arm/arm64/entry.S
> @@ -191,6 +191,7 @@
>          ALTERNATIVE("bl check_pending_vserror; cbnz x0, 1f",
>                      "nop; nop",
>                      SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT)
> +        bl      enter_hypervisor_from_guest_preirq
>          msr     daifclr, \iflags
>          bl      enter_hypervisor_from_guest
>          mov     x0, sp
> diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
> index adbedc2d15..cb4e3b627b 100644
> --- a/xen/arch/arm/traps.c
> +++ b/xen/arch/arm/traps.c
> @@ -1986,15 +1986,25 @@ static inline bool needs_ssbd_flip(struct vcpu *v)
>  
>  /*
>   * Actions that needs to be done after entering the hypervisor from the
> - * guest and before we handle any request.
> + * guest and before the interrupts are unmasked.
>   */
> -void enter_hypervisor_from_guest(void)
> +void enter_hypervisor_from_guest_preirq(void)
>  {
>      struct vcpu *v = current;
>  
>      /* If the guest has disabled the workaround, bring it back on. */
>      if ( needs_ssbd_flip(v) )
>          arm_smccc_1_1_smc(ARM_SMCCC_ARCH_WORKAROUND_2_FID, 1, NULL);
> +}
> +
> +/*
> + * Actions that needs to be done after entering the hypervisor from the
> + * guest and before we handle any request. Depending on the exception trap,
> + * this may be called with interrupts unmasked.
> + */
> +void enter_hypervisor_from_guest(void)
> +{
> +    struct vcpu *v = current;
>  
>      /*
>       * If we pended a virtual abort, preserve it until it gets cleared.
> -- 
> 2.11.0
> 

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