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

[Michael Ellerman]

Balbir Singh <bsingharora@xxxxxxxxx> writes:
> On Sat, Feb 2, 2019 at 12:14 PM Balbir Singh <bsingharora@xxxxxxxxx> wrote:
>>
>> On Tue, Jan 22, 2019 at 10:57:21AM -0500, Joe Lawrence wrote:
>> > From: Nicolai Stange <nstange@xxxxxxx>
>> >
>> > 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@xxxxxxxxxx>
>> > Signed-off-by: Nicolai Stange <nstange@xxxxxxx>
>> > Signed-off-by: Joe Lawrence <joe.lawrence@xxxxxxxxxx>
>> > ---
>> >  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)
>>
>
> r4 is not 0, sorry for the noise

Isn't it?

cheers