4.19-stable review patch. If anyone has any objections, please let me know.
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[ 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 --- arch/powerpc/kernel/entry_64.S | 7 +++++++ 1 file changed, 7 insertions(+)
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S index c806a3c12592..7a46e0e57a36 100644 --- a/arch/powerpc/kernel/entry_64.S +++ b/arch/powerpc/kernel/entry_64.S @@ -994,6 +994,13 @@ 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)