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

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Date:   Sat, 8 Feb 2020 07:54:39 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Kees Cook <keescook@...omium.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, 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>

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.


> ---
> 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