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

[Michael Ellerman]

Russell Currey <ruscur at russell.cc> writes:
> 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")

Don't word wrap the fixes line please.

> Cc: <stable at vger.kernel.org>
> Signed-off-by: Russell Currey <ruscur at russell.cc>
> ---
>  arch/powerpc/include/asm/cpuidle.h |  1 +
>  arch/powerpc/kernel/asm-offsets.c  |  1 +
>  arch/powerpc/kernel/idle_book3s.S  | 20 ++++++++++++++++++++
>  3 files changed, 22 insertions(+)
>
> diff --git a/arch/powerpc/include/asm/cpuidle.h b/arch/powerpc/include/asm/cpuidle.h
> index 43e5f31fe64d..ad67dbe59498 100644
> --- a/arch/powerpc/include/asm/cpuidle.h
> +++ b/arch/powerpc/include/asm/cpuidle.h
> @@ -77,6 +77,7 @@ struct stop_sprs {
>  	u64 mmcr1;
>  	u64 mmcr2;
>  	u64 mmcra;
> +	u64 iamr;
>  };

We don't actually need to put this in the paca anymore.

> diff --git a/arch/powerpc/kernel/idle_book3s.S b/arch/powerpc/kernel/idle_book3s.S
> index 7f5ac2e8581b..bb4f552f6c7e 100644
> --- a/arch/powerpc/kernel/idle_book3s.S
> +++ b/arch/powerpc/kernel/idle_book3s.S
> @@ -200,6 +200,12 @@ pnv_powersave_common:
>  	/* Continue saving state */
>  	SAVE_GPR(2, r1)
>  	SAVE_NVGPRS(r1)
> +
> +BEGIN_FTR_SECTION
> +	mfspr	r5, SPRN_IAMR
> +	std	r5, STOP_IAMR(r13)
> +END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)

We have space for a full pt_regs on the stack, and we're not using it
all.

We don't have a specific slot for the IAMR (we may want to in future),
but for now you could follow the time-honoured tradition of (ab)using
the _DAR slot, with an appropriate comment.

cheers