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

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Date:   Sat, 8 Feb 2020 07:18:47 +0000
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Nicolas Pitre <nico@...xnic.net>,
        Manoj Gupta <manojgupta@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] ARM: rename missed uaccess .fixup section

On Sat, Feb 8, 2020 at 3:02 AM Kees Cook <keescook@...omium.org> wrote:

Looks like we were both up late debugging this! Great job finding a fix!

>
> 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")

I was curious if the "mix" of `.fixup` and `.text.fixup` in a few
places under arch/arm/ was intentional or not. I should have
investigated that more.

> 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>

Before this patch:
$ ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- make CC=clang LD=ld.lld -j71
$ readelf -S arch/arm/lib/copy_from_user.o
...
   [ 9] .fixup            PROGBITS        00000000 0004e8 00001c 00
AX  0   0  4
...
$ readelf -S vmlinux
...
  [ 2] .fixup            PROGBITS        c020826c 00126c 00001c 00  AX  0   0  4
  [ 3] .text             PROGBITS        c0300000 002000 d71964 00 WAX
 0   0 4096
....
(Which is bad since .fixup resides before _stext!)
$ readelf -s vmlinux | grep _stext
203324: c0300000     0 NOTYPE  GLOBAL DEFAULT    3 _stext

After this patch is applied:
$ readelf -S arch/arm/lib/copy_from_user.o
...
  [ 9] .text.fixup       PROGBITS        00000000 0004e8 00001c 00  AX  0   0  4
...
$ readelf -S vmlinux
...
  [ 2] .text             PROGBITS        c0300000 002000 d71964 00 WAX
 0   0 4096
...
(So there's no orphaned .fixup section).  I forget if I was just
discussing it w/ Ard or Arnd a few days ago but I think we should
really enable warning on orphan sections during link (lest we continue
to run into issues like this).

$ grep -r \\.fixup arch/arm

turns up another hit in:
arch/arm/boot/compressed/vmlinux.lds.S:    *(.fixup)

Which I think should be fixed, too, maybe in a V2 so I'll hold my
reviewed-by tag for now. (Modifying that locally, I'm able to boot
qemu, and I also don't see any object files with such a section, ie.

$ readelf -S arch/arm/boot/compressed/*.o | grep fixup

comes up empty. So it could be renamed or even removed).

We should also cc stable, since c4a84ae39b4a5 first landed in v4.1-rc1.

Thanks for the patch!

> ---
> 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



-- 
Thanks,
~Nick Desaulniers