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Date: Fri, 23 Feb 2024 19:26:46 +0100
From: Sam Ravnborg <sam@ravnborg.org>
To: Kees Cook <keescook@chromium.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: <20240223182646.GA2825372@ravnborg.org>
References: <20240223165942.work.950-kees@kernel.org>
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In-Reply-To: <20240223165942.work.950-kees@kernel.org>

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.

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?

	Sam