linux-kernel - Re: [PATCH v2 02/28] x86/asm: Replace __force

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Date:   Thu, 3 Sep 2020 14:45:54 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        x86@...nel.org, Arvind Sankar <nivedita@...m.mit.edu>
Subject: Re: [PATCH v2 02/28] x86/asm: Replace __force_order with memory
 clobber

On Thu, Sep 03, 2020 at 01:30:27PM -0700, Sami Tolvanen wrote:
> From: Arvind Sankar <nivedita@...m.mit.edu>
> 
> The CRn accessor functions use __force_order as a dummy operand to
> prevent the compiler from reordering CRn reads/writes with respect to
> each other.
> 
> The fact that the asm is volatile should be enough to prevent this:
> volatile asm statements should be executed in program order. However GCC
> 4.9.x and 5.x have a bug that might result in reordering. This was fixed
> in 8.1, 7.3 and 6.5. Versions prior to these, including 5.x and 4.9.x,
> may reorder volatile asm statements with respect to each other.
> 
> There are some issues with __force_order as implemented:
> - It is used only as an input operand for the write functions, and hence
>   doesn't do anything additional to prevent reordering writes.
> - It allows memory accesses to be cached/reordered across write
>   functions, but CRn writes affect the semantics of memory accesses, so
>   this could be dangerous.
> - __force_order is not actually defined in the kernel proper, but the
>   LLVM toolchain can in some cases require a definition: LLVM (as well
>   as GCC 4.9) requires it for PIE code, which is why the compressed
>   kernel has a definition, but also the clang integrated assembler may
>   consider the address of __force_order to be significant, resulting in
>   a reference that requires a definition.
> 
> Fix this by:
> - Using a memory clobber for the write functions to additionally prevent
>   caching/reordering memory accesses across CRn writes.
> - Using a dummy input operand with an arbitrary constant address for the
>   read functions, instead of a global variable. This will prevent reads
>   from being reordered across writes, while allowing memory loads to be
>   cached/reordered across CRn reads, which should be safe.
> 
> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>

In the primary thread for this patch I sent a Reviewed tag, but for good
measure, here it is again:

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook