Re: [PATCH 1/4] powerpc/64s: Clear on-stack exception marker upon exception return

[Balbir Singh <bsingharora@gmail.com>]

On Tue, Jan 22, 2019 at 10:57:21AM -0500, Joe Lawrence wrote:
> From: Nicolai Stange <nstange@suse.de>
> 
> The ppc64 specific implementation of the reliable stacktracer,
> save_stack_trace_tsk_reliable(), bails out and reports an "unreliable
> trace" whenever it finds an exception frame on the stack. Stack frames
> are classified as exception frames if the STACK_FRAME_REGS_MARKER magic,
> as written by exception prologues, is found at a particular location.
> 
> However, as observed by Joe Lawrence, it is possible in practice that
> non-exception stack frames can alias with prior exception frames and thus,
> that the reliable stacktracer can find a stale STACK_FRAME_REGS_MARKER on
> the stack. It in turn falsely reports an unreliable stacktrace and blocks
> any live patching transition to finish. Said condition lasts until the
> stack frame is overwritten/initialized by function call or other means.
> 
> In principle, we could mitigate this by making the exception frame
> classification condition in save_stack_trace_tsk_reliable() stronger:
> in addition to testing for STACK_FRAME_REGS_MARKER, we could also take into
> account that for all exceptions executing on the kernel stack
> - their stack frames's backlink pointers always match what is saved
>   in their pt_regs instance's ->gpr[1] slot and that
> - their exception frame size equals STACK_INT_FRAME_SIZE, a value
>   uncommonly large for non-exception frames.
> 
> However, while these are currently true, relying on them would make the
> reliable stacktrace implementation more sensitive towards future changes in
> the exception entry code. Note that false negatives, i.e. not detecting
> exception frames, would silently break the live patching consistency model.
> 
> Furthermore, certain other places (diagnostic stacktraces, perf, xmon)
> rely on STACK_FRAME_REGS_MARKER as well.
> 
> Make the exception exit code clear the on-stack STACK_FRAME_REGS_MARKER
> for those exceptions running on the "normal" kernel stack and returning
> to kernelspace: because the topmost frame is ignored by the reliable stack
> tracer anyway, returns to userspace don't need to take care of clearing
> the marker.
> 
> Furthermore, as I don't have the ability to test this on Book 3E or
> 32 bits, limit the change to Book 3S and 64 bits.
> 
> Finally, make the HAVE_RELIABLE_STACKTRACE Kconfig option depend on
> PPC_BOOK3S_64 for documentation purposes. Before this patch, it depended
> on PPC64 && CPU_LITTLE_ENDIAN and because CPU_LITTLE_ENDIAN implies
> PPC_BOOK3S_64, there's no functional change here.
> 
> Fixes: df78d3f61480 ("powerpc/livepatch: Implement reliable stack tracing for the consistency model")
> Reported-by: Joe Lawrence <joe.lawrence@redhat.com>
> Signed-off-by: Nicolai Stange <nstange@suse.de>
> Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
> ---
>  arch/powerpc/Kconfig           | 2 +-
>  arch/powerpc/kernel/entry_64.S | 7 +++++++
>  2 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 2890d36eb531..73bf87b1d274 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -220,7 +220,7 @@ config PPC
>  	select HAVE_PERF_USER_STACK_DUMP
>  	select HAVE_RCU_TABLE_FREE		if SMP
>  	select HAVE_REGS_AND_STACK_ACCESS_API
> -	select HAVE_RELIABLE_STACKTRACE		if PPC64 && CPU_LITTLE_ENDIAN
> +	select HAVE_RELIABLE_STACKTRACE		if PPC_BOOK3S_64 && CPU_LITTLE_ENDIAN
>  	select HAVE_SYSCALL_TRACEPOINTS
>  	select HAVE_VIRT_CPU_ACCOUNTING
>  	select HAVE_IRQ_TIME_ACCOUNTING
> diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
> index 435927f549c4..a2c168b395d2 100644
> --- a/arch/powerpc/kernel/entry_64.S
> +++ b/arch/powerpc/kernel/entry_64.S
> @@ -1002,6 +1002,13 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
>  	ld	r2,_NIP(r1)
>  	mtspr	SPRN_SRR0,r2
>  
> +	/*
> +	 * Leaving a stale exception_marker on the stack can confuse
> +	 * the reliable stack unwinder later on. Clear it.
> +	 */
> +	li	r2,0
> +	std	r2,STACK_FRAME_OVERHEAD-16(r1)
> +

Could you please double check, r4 is already 0 at this point
IIUC. So the change might be a simple

std r4,STACK_FRAME_OVERHEAD-16(r1)

Balbir