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

[Joe Lawrence <joe.lawrence@redhat.com>]

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)
+
 	ld	r0,GPR0(r1)
 	ld	r2,GPR2(r1)
 	ld	r3,GPR3(r1)
-- 
2.20.1