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Date:   Thu, 3 Sep 2020 14:44:41 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        clang-built-linux@googlegroups.com,
        kernel-hardening@lists.openwall.com, linux-arch@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-kbuild@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org,
        x86@kernel.org, Arvind Sankar <nivedita@alum.mit.edu>
Subject: Re: [PATCH v2 01/28] x86/boot/compressed: Disable relocation
 relaxation
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 <20200903203053.3411268-1-samitolvanen@google.com>
 <20200903203053.3411268-2-samitolvanen@google.com>
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On Thu, Sep 03, 2020 at 01:30:26PM -0700, Sami Tolvanen wrote:
> From: Arvind Sankar <nivedita@alum.mit.edu>
> 
> The x86-64 psABI [0] specifies special relocation types
> (R_X86_64_[REX_]GOTPCRELX) for indirection through the Global Offset
> Table, semantically equivalent to R_X86_64_GOTPCREL, which the linker
> can take advantage of for optimization (relaxation) at link time. This
> is supported by LLD and binutils versions 2.26 onwards.
> 
> The compressed kernel is position-independent code, however, when using
> LLD or binutils versions before 2.27, it must be linked without the -pie
> option. In this case, the linker may optimize certain instructions into
> a non-position-independent form, by converting foo@GOTPCREL(%rip) to $foo.
> 
> This potential issue has been present with LLD and binutils-2.26 for a
> long time, but it has never manifested itself before now:
> - LLD and binutils-2.26 only relax
> 	movq	foo@GOTPCREL(%rip), %reg
>   to
> 	leaq	foo(%rip), %reg
>   which is still position-independent, rather than
> 	mov	$foo, %reg
>   which is permitted by the psABI when -pie is not enabled.
> - gcc happens to only generate GOTPCREL relocations on mov instructions.
> - clang does generate GOTPCREL relocations on non-mov instructions, but
>   when building the compressed kernel, it uses its integrated assembler
>   (due to the redefinition of KBUILD_CFLAGS dropping -no-integrated-as),
>   which has so far defaulted to not generating the GOTPCRELX
>   relocations.
> 
> Nick Desaulniers reports [1,2]:
>   A recent change [3] to a default value of configuration variable
>   (ENABLE_X86_RELAX_RELOCATIONS OFF -> ON) in LLVM now causes Clang's
>   integrated assembler to emit R_X86_64_GOTPCRELX/R_X86_64_REX_GOTPCRELX
>   relocations. LLD will relax instructions with these relocations based
>   on whether the image is being linked as position independent or not.
>   When not, then LLD will relax these instructions to use absolute
>   addressing mode (R_RELAX_GOT_PC_NOPIC). This causes kernels built with
>   Clang and linked with LLD to fail to boot.
> 
> Patch series [4] is a solution to allow the compressed kernel to be
> linked with -pie unconditionally, but even if merged is unlikely to be
> backported. As a simple solution that can be applied to stable as well,
> prevent the assembler from generating the relaxed relocation types using
> the -mrelax-relocations=no option. For ease of backporting, do this
> unconditionally.
> 
> [0] https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/linker-optimization.tex#L65
> [1] https://lore.kernel.org/lkml/20200807194100.3570838-1-ndesaulniers@google.com/
> [2] https://github.com/ClangBuiltLinux/linux/issues/1121
> [3] https://reviews.llvm.org/rGc41a18cf61790fc898dcda1055c3efbf442c14c0
> [4] https://lore.kernel.org/lkml/20200731202738.2577854-1-nivedita@alum.mit.edu/
> 
> Reported-by: Nick Desaulniers <ndesaulniers@google.com>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>

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

-- 
Kees Cook