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Date: Fri, 23 Feb 2024 15:32:37 -0800
From: Kees Cook <keescook@chromium.org>
To: Sam Ravnborg <sam@ravnborg.org>
Cc: "David S. Miller" <davem@davemloft.net>,
	Andreas Larsson <andreas@gaisler.com>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Helge Deller <deller@gmx.de>, Guo Ren <guoren@kernel.org>,
	sparclinux@vger.kernel.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	"Russell King (Oracle)" <rmk+kernel@armlinux.org.uk>,
	linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH] sparc: vdso: Disable UBSAN instrumentation
Message-ID: <202402231523.F9E3740EB@keescook>
References: <20240223165942.work.950-kees@kernel.org>
 <20240223182646.GA2825372@ravnborg.org>
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In-Reply-To: <20240223182646.GA2825372@ravnborg.org>

On Fri, Feb 23, 2024 at 07:26:46PM +0100, Sam Ravnborg wrote:
> Hi Kees,
> 
> On Fri, Feb 23, 2024 at 08:59:45AM -0800, Kees Cook wrote:
> > The UBSAN instrumentation cannot work in the vDSO since it is executing
> > in userspace, so disable it in the Makefile. Fixes the build failures
> > such as:
> > 
> > arch/sparc/vdso/vclock_gettime.c:217: undefined reference to `__ubsan_handle_shift_out_of_bounds'
> > 
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> > Cc: "David S. Miller" <davem@davemloft.net>
> > Cc: Andreas Larsson <andreas@gaisler.com>
> > Cc: Masahiro Yamada <masahiroy@kernel.org>
> > Cc: Sam Ravnborg <sam@ravnborg.org>
> > Cc: Helge Deller <deller@gmx.de>
> > Cc: Guo Ren <guoren@kernel.org>
> > Cc: sparclinux@vger.kernel.org
> > ---
> >  arch/sparc/vdso/Makefile | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/sparc/vdso/Makefile b/arch/sparc/vdso/Makefile
> > index 7f5eedf1f5e0..e8aef2c8ae99 100644
> > --- a/arch/sparc/vdso/Makefile
> > +++ b/arch/sparc/vdso/Makefile
> > @@ -2,6 +2,7 @@
> >  #
> >  # Building vDSO images for sparc.
> >  #
> > +UBSAN_SANITIZE := n
> 
> When I read:
> 
> config UBSAN_SANITIZE_ALL
>         bool "Enable instrumentation for the entire kernel"
>         depends on ARCH_HAS_UBSAN_SANITIZE_ALL
>         default y
>         help
>           This option activates instrumentation for the entire kernel.
>           If you don't enable this option, you have to explicitly specify
>           UBSAN_SANITIZE := y for the files/directories you want to check for UB.
>           Enabling this option will get kernel image size increased
>           significantly.
> 
> 
> I am left with the understanding that only arch's that
> selects ARCH_HAS_UBSAN_SANITIZE_ALL would need to turn off
> UBSAN_SANITIZE.

Ah, right. So, I removed[1] UBSAN_SANITIZE_ALL in -next (it was the only
sanitizer using this logic) and this appears to be one of the impacts. :)
I sent similar fixes for sh[2] and LoongArch[3].

> Are this fix papering over some other bug where we enable
> UBSAN_SANITIZE_ALL for arch's that should not have it,
> or something else that enable it?

It's possible we should implement HAVE_ARCH_UBSAN, but in my testing
everything built fine with it, so I didn't opt to do that (it looked
like just additional configs for no real benefit). What do you think?

-Kees

[1] https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=for-next/kspp&id=918327e9b7ffb45321cbb4b9b86b58ec555fe6b3
[2] https://lore.kernel.org/lkml/20240130232717.work.088-kees@kernel.org/
[3] https://lore.kernel.org/lkml/20240130233140.work.887-kees@kernel.org/

-- 
Kees Cook