powerpc/64s: Clear on-stack exception marker upon exception return

BugLink: https://bugs.launchpad.net/bugs/1838116
[ Upstream commit eddd0b332304d554ad6243942f87c2fcea98c56b ]

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.

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>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index b9cc8f0fd7bcb2ee9c8156d8d178c8f584115bba..5e507c1e07cd6e2c16c71b287f1cd45976b93405 100644 (file)
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -985,6 +985,13 @@ 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)