Re: [PATCH edk2-stable202305] ArmPkg/ArmMmuLib: Add missing ISB after page table update

["Leif Lindholm" <quic_llindhol@quicinc.com>]

On Sat, May 20, 2023 at 11:19:58 +0200, Ard Biesheuvel wrote:
> The helper that updates live page table entries writes a zero entry,
> invalidates the covered address range from the TLBs, and finally writes
> the actual entry. This ensures that no TLB conflicts can occur.
> 
> Writing the final entry needs to complete before any translations can be
> performed, as otherwise, the zero entry, which describes an invalid
> translation, may be observed by the page table walker, resulting in a
> translation fault. For this reason, the final write is followed by a DSB
> barrier instruction.
> 
> However, this barrier will not stall the pipeline, and instruction
> fetches may still hit this invalid translation, as has been observed and
> reported by Oliver. To ensure that the new translation is fully active
> before returning from this helper, we have to insert an ISB barrier as
> well.
> 
> Cc: Liming Gao <gaoliming@byosoft.com.cn>
> Cc: Leif Lindholm <quic_llindhol@quicinc.com>
> Cc: Michael D Kinney <michael.d.kinney@intel.com>
> Reported-by: Oliver Steffen <osteffen@redhat.com>
> Tested-by: Oliver Steffen <osteffen@redhat.com>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>

We need this in the stable tag.

Note: the isb instruction forces the synchronization of certain
architectural events. It has no other effects. I.e., any issues
exposed by this addition would already have been present before it.

As such, I would suggest this addition need *not* affect the stable
tag schedule.

/
    Leif

> ---
>  ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibReplaceEntry.S | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibReplaceEntry.S b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibReplaceEntry.S
> index 887439bc042f0f16..1f0d8057926933d7 100644
> --- a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibReplaceEntry.S
> +++ b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibReplaceEntry.S
> @@ -65,6 +65,7 @@
>    // write updated entry
>    str   x1, [x0]
>    dsb   nshst
> +  isb
>  
>  .L2_\@:
>    .endm
> -- 
> 2.39.2
>