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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id d77sm4259177pfd.121.2020.09.03.14.44.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 03 Sep 2020 14:44:42 -0700 (PDT) Date: Thu, 3 Sep 2020 14:44:41 -0700 From: Kees Cook To: Sami Tolvanen Cc: Masahiro Yamada , Will Deacon , Peter Zijlstra , Steven Rostedt , Greg Kroah-Hartman , "Paul E. McKenney" , Nick Desaulniers , 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 Subject: Re: [PATCH v2 01/28] x86/boot/compressed: Disable relocation relaxation Message-ID: <202009031444.F2ECA89E@keescook> References: <20200624203200.78870-1-samitolvanen@google.com> <20200903203053.3411268-1-samitolvanen@google.com> <20200903203053.3411268-2-samitolvanen@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200903203053.3411268-2-samitolvanen@google.com> Sender: linux-arch-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On Thu, Sep 03, 2020 at 01:30:26PM -0700, Sami Tolvanen wrote: > From: Arvind Sankar > > 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 > Signed-off-by: Arvind Sankar Reviewed-by: Kees Cook -- Kees Cook