Messages in this thread Patch in this message | | | From | Nick Desaulniers <> | Date | Sat, 8 Feb 2020 07:18:47 +0000 | Subject | Re: [PATCH] ARM: rename missed uaccess .fixup section |
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On Sat, Feb 8, 2020 at 3:02 AM Kees Cook <keescook@chromium.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@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>
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
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