linux-kernel - Re: [PATCH] ARM: rename missed uaccess .fixup section

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Date:   Sat, 8 Feb 2020 00:55:10 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Nicolas Pitre <nico@...xnic.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Manoj Gupta <manojgupta@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: rename missed uaccess .fixup section

On Sat, Feb 08, 2020 at 07:54:39AM +0000, Ard Biesheuvel wrote:
> On Sat, 8 Feb 2020 at 02:02, Kees Cook <keescook@...omium.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@...r.kernel.org
> > Reported-by: Nathan Chancellor <natechancellor@...il.com>
> > Reported-by: Manoj Gupta <manojgupta@...gle.com>
> > Debugged-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> 
> Reviewed-by: Ard Biesheuvel <ardb@...nel.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.

-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