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

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Message-ID: <CAKwvOdkNLK2cRfY+DA0u3KuY5PuEss5Qox6X3zzpL053pARfrA@mail.gmail.com>
Date:   Mon, 10 Aug 2020 15:33:22 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Huckleberry <nhuck15@...il.com>
Cc:     Russell King <linux@...linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chunyan Zhang <zhang.lyra@...il.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mediatek@...ts.infradead.org,
        Lvqiang Huang <lvqiang.huang@...soc.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Miles Chen <miles.chen@...iatek.com>,
        "# 3.4.x" <stable@...r.kernel.org>,
        Nathan Huckleberry <nhuck@...gle.com>
Subject: Re: [PATCH 2/4] ARM: backtrace-clang: add fixup for lr dereference

On Thu, Aug 6, 2020 at 3:39 PM Nathan Huckleberry <nhuck15@...il.com> wrote:
>
> Mostly looks good to me. Just a minor nit.
>
> On Thu, Jul 30, 2020 at 3:51 PM Nick Desaulniers
> <ndesaulniers@...gle.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@...r.kernel.org
> > Fixes: commit 6dc5fd93b2f1 ("ARM: 8900/1: UNWINDER_FRAME_POINTER implementation for Clang")
> > Reported-by: Miles Chen <miles.chen@...iatek.com>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.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