Re: [PATCH v2] powerpc/powernv/idle: Restore IAMR after idle

[Gautham R Shenoy <ego@linux.vnet.ibm.com>]

Hello Russell,

On Fri, Feb 08, 2019 at 10:11:03PM +1100, Russell Currey wrote:
> Without restoring the IAMR after idle, execution prevention on POWER9
> with Radix MMU is overwritten and the kernel can freely execute userspace without
> faulting.
> 
> This is necessary when returning from any stop state that modifies user
> state, as well as hypervisor state.
> 
> To test how this fails without this patch, load the lkdtm driver and
> do the following:
> 
>    echo EXEC_USERSPACE > /sys/kernel/debug/provoke-crash/DIRECT
> 
> which won't fault, then boot the kernel with powersave=off, where it
> will fault.  Applying this patch will fix this.
> 
> Fixes: 3b10d0095a1e ("powerpc/mm/radix: Prevent kernel execution of user
> space")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Russell Currey <ruscur@russell.cc>
> ---
> Since v1:
>       - no longer use paca to save IAMR, instead use _DAR (thanks mpe)

Looks good to me. Once we move to Nick Piggin's C-based save/restore
code, we will be saving all these SPR values on the stack anyway.

Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>

--
Thanks and Regards
gautham.