[PATCH 2/4] ARM: backtrace-clang: add fixup for lr dereference

[Nick Desaulniers]

On Thu, Aug 6, 2020 at 3:39 PM Nathan Huckleberry <nhuck15 at gmail.com> wrote:
>
> Mostly looks good to me. Just a minor nit.
>
> On Thu, Jul 30, 2020 at 3:51 PM Nick Desaulniers
> <ndesaulniers at google.com> wrote:
> >
> > If the value of the link register is not correct (tail call from asm
> > that didn't set it, stack corruption, memory no longer mapped), then
> > using it for an address calculation may trigger an exception.  Without a
> > fixup handler, this will lead to a panic, which will unwind, which will
> > trigger the fault repeatedly in an infinite loop.
> >
> > We don't observe such failures currently, but we have. Just to be safe,
> > add a fixup handler here so that at least we don't have an infinite
> > loop.
> >
> > Cc: stable at vger.kernel.org
> > Fixes: commit 6dc5fd93b2f1 ("ARM: 8900/1: UNWINDER_FRAME_POINTER implementation for Clang")
> > Reported-by: Miles Chen <miles.chen at mediatek.com>
> > Signed-off-by: Nick Desaulniers <ndesaulniers at google.com>
> > ---
> >  arch/arm/lib/backtrace-clang.S | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm/lib/backtrace-clang.S b/arch/arm/lib/backtrace-clang.S
> > index 5388ac664c12..40eb2215eaf4 100644
> > --- a/arch/arm/lib/backtrace-clang.S
> > +++ b/arch/arm/lib/backtrace-clang.S
> > @@ -146,7 +146,7 @@ for_each_frame:     tst     frame, mask             @ Check for address exceptions
> >
> >                 tst     sv_lr, #0               @ If there's no previous lr,
> >                 beq     finished_setup          @ we're done.
> > -               ldr     r0, [sv_lr, #-4]        @ get call instruction
> > +prev_call:     ldr     r0, [sv_lr, #-4]        @ get call instruction
> >                 ldr     r3, .Lopcode+4
> >                 and     r2, r3, r0              @ is this a bl call
> >                 teq     r2, r3
> > @@ -206,6 +206,13 @@ finished_setup:
> >                 mov     r2, frame
> >                 bl      printk
> >  no_frame:      ldmfd   sp!, {r4 - r9, fp, pc}
> > +/*
> > + * Accessing the address pointed to by the link register triggered an
> > + * exception, don't try to unwind through it.
> > + */
> > +bad_lr:                mov     sv_fp, #0
>
> It might be nice to emit a warning here since we'll
> only hit this case if something fishy is going on
> with the saved lr.

Yeah, something fishy is going on if that ever happens.  Let me create
a V2 with an additional print.

>
> > +               mov     sv_lr, #0
> > +               b       finished_setup
> >  ENDPROC(c_backtrace)
> >                 .pushsection __ex_table,"a"
> >                 .align  3
> > @@ -214,6 +221,7 @@ ENDPROC(c_backtrace)
> >                 .long   1003b, 1006b
> >                 .long   1004b, 1006b
> >                 .long   1005b, 1006b
> > +               .long   prev_call, bad_lr
> >                 .popsection
> >
> >  .Lbad:         .asciz  "%sBacktrace aborted due to bad frame pointer <%p>\n"
> > --
> > 2.28.0.163.g6104cc2f0b6-goog
> >
>
> Thanks,
> Huck



-- 
Thanks,
~Nick Desaulniers