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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Sat, 8 Feb 2020 10:04:53 +0000
Message-ID: <CAKwvOdmchUkLLQMjagJ1cxa3CwFaAgH8gZcMVNny9Fb0iDVi8A@mail.gmail.com>
Subject: Re: [PATCH] ARM: rename missed uaccess .fixup section
To:     Kees Cook <keescook@chromium.org>
Cc:     Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Nicolas Pitre <nico@fluxnic.net>,
        Manoj Gupta <manojgupta@google.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Russell King - ARM Linux <linux@armlinux.org.uk>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Sat, Feb 8, 2020 at 9:55 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Sat, Feb 08, 2020 at 07:54:39AM +0000, Ard Biesheuvel wrote:
> > On Sat, 8 Feb 2020 at 02:02, Kees Cook <keescook@chromium.org> wrote:
> > >
> > > When the uaccess .fixup section was renamed to .text.fixup, one case was
> > > missed. Under ld.bfd, the orphaned section was moved close to .text
> > > (since they share the "ax" bits), so things would work normally on
> > > uaccess faults. Under ld.lld, the orphaned section was placed outside
> > > the .text section, making it unreachable. Rename the missed section.
> > >
> > > Link: https://github.com/ClangBuiltLinux/linux/issues/282
> > > Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1020633#c44
> > > Link: https://lore.kernel.org/r/nycvar.YSQ.7.76.1912032147340.17114@knanqh.ubzr
> > > Fixes: c4a84ae39b4a5 ("ARM: 8322/1: keep .text and .fixup regions closer together")
> > > Cc: stable@vger.kernel.org
> > > Reported-by: Nathan Chancellor <natechancellor@gmail.com>
> > > Reported-by: Manoj Gupta <manojgupta@google.com>
> > > Debugged-by: Nick Desaulniers <ndesaulniers@google.com>
> > > Signed-off-by: Kees Cook <keescook@chromium.org>
> >
> > Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
>
> Thanks!
>
> > As Nick points out, the *(.fixup) line still appears in the
> > decompressor's linker script, but this is harmless, given that we
> > don't ever emit anything into that section. But while we're at it, we
> > might just remove it as well.
>
> Agreed. I'll send a separate patch for that.

Sure, in that case
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

>
> -Kees
>
> >
> >
> > > ---
> > > I completely missed this the first several times I looked at this
> > > problem. Thank you Nicolas for pushing back on the earlier patch!
> > > Manoj or Nathan, can you test this?
> > > ---
> > >  arch/arm/lib/copy_from_user.S | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/arch/arm/lib/copy_from_user.S b/arch/arm/lib/copy_from_user.S
> > > index 95b2e1ce559c..f8016e3db65d 100644
> > > --- a/arch/arm/lib/copy_from_user.S
> > > +++ b/arch/arm/lib/copy_from_user.S
> > > @@ -118,7 +118,7 @@ ENTRY(arm_copy_from_user)
> > >
> > >  ENDPROC(arm_copy_from_user)
> > >
> > > -       .pushsection .fixup,"ax"
> > > +       .pushsection .text.fixup,"ax"
> > >         .align 0
> > >         copy_abort_preamble
> > >         ldmfd   sp!, {r1, r2, r3}
> > > --
> > > 2.20.1
> > >
> > >
> > > --
> > > Kees Cook
>
> --
> Kees Cook


-- 
Thanks,
~Nick Desaulniers