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

[Nicholas Piggin]

Russell Currey's on February 6, 2019 4:28 pm:
> 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 at vger.kernel.org>
> Signed-off-by: Russell Currey <ruscur at russell.cc>

Good catch and debugging. This really should be a quirk, we don't want 
to have to restore this thing on a thread switch.

Can we put it under a CONFIG option if we're not using IAMR?

> ---
>  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;
>  };
>  
>  #define PNV_IDLE_NAME_LEN    16
> diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
> index 9ffc72ded73a..10e0314c2b0d 100644
> --- a/arch/powerpc/kernel/asm-offsets.c
> +++ b/arch/powerpc/kernel/asm-offsets.c
> @@ -774,6 +774,7 @@ int main(void)
>  	STOP_SPR(STOP_MMCR1, mmcr1);
>  	STOP_SPR(STOP_MMCR2, mmcr2);
>  	STOP_SPR(STOP_MMCRA, mmcra);
> +	STOP_SPR(STOP_IAMR, iamr);
>  #endif
>  
>  	DEFINE(PPC_DBELL_SERVER, PPC_DBELL_SERVER);
> 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)
> +
>  	mfcr	r5
>  	std	r5,_CCR(r1)
>  	std	r1,PACAR1(r13)
> @@ -924,6 +930,13 @@ BEGIN_FTR_SECTION
>  END_FTR_SECTION_IFSET(CPU_FTR_HVMODE)
>  	REST_NVGPRS(r1)
>  	REST_GPR(2, r1)
> +
> +BEGIN_FTR_SECTION
> +	ld	r4, STOP_IAMR(r13)
> +	mtspr	SPRN_IAMR, r4
> +	isync
> +END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)

Sigh, good old isync. Suspect you'll get away without it, mtmsrd L=0 
just below is architecturally guaranteeing a CSI, so just add a comment 
there, might save a flush.

> +
>  	ld	r4,PACAKMSR(r13)
>  	ld	r5,_LINK(r1)
>  	ld	r6,_CCR(r1)
> @@ -946,6 +959,13 @@ pnv_wakeup_noloss:
>  BEGIN_FTR_SECTION
>  	CHECK_HMI_INTERRUPT
>  END_FTR_SECTION_IFSET(CPU_FTR_HVMODE)
> +
> +BEGIN_FTR_SECTION
> +	ld	r4, STOP_IAMR(r13)
> +	mtspr	SPRN_IAMR, r4
> +	isync
> +END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)

For the noloss part, it should mean really nothing lost including GPRs, 
so I think IAMR *should* be okay here.

Thanks,
Nick