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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id o65sm13181259pfg.187.2020.03.31.12.46.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 31 Mar 2020 12:46:34 -0700 (PDT) Date: Tue, 31 Mar 2020 12:46:33 -0700 From: Kees Cook To: Mark Brown Cc: Catalin Marinas , Will Deacon , Mark Rutland , Amit Daniel Kachhap , linux-arm-kernel@lists.infradead.org, Szabolcs Nagy , stable@vger.kernel.org Subject: Re: [PATCH] arm64: Always force a branch protection mode when the compiler has one Message-ID: <202003311246.3BAE76AF1@keescook> References: <20200331194459.54740-1-broonie@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200331194459.54740-1-broonie@kernel.org> Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On Tue, Mar 31, 2020 at 08:44:59PM +0100, Mark Brown wrote: > Compilers with branch protection support can be configured to enable it by > default, it is likely that distributions will do this as part of deploying > branch protection system wide. As well as the slight overhead from having > some extra NOPs for unused branch protection features this can cause more > serious problems when the kernel is providing pointer authentication to > userspace but not built for pointer authentication itself. In that case our > switching of keys for userspace can affect the kernel unexpectedly, causing > pointer authentication instructions in the kernel to corrupt addresses. > > To ensure that we get consistent and reliable behaviour always explicitly > initialise the branch protection mode, ensuring that the kernel is built > the same way regardless of the compiler defaults. > > Fixes: 7503197562567 (arm64: add basic pointer authentication support) > Reported-by: Szabolcs Nagy > Signed-off-by: Mark Brown Reviewed-by: Kees Cook -Kees > Cc: stable@vger.kernel.org > --- > arch/arm64/Kconfig | 4 ++++ > arch/arm64/Makefile | 7 ++++++- > 2 files changed, 10 insertions(+), 1 deletion(-) > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig > index d3efdc095a17..1e46746e8392 100644 > --- a/arch/arm64/Kconfig > +++ b/arch/arm64/Kconfig > @@ -1537,6 +1537,10 @@ config ARM64_PTR_AUTH > This feature works with FUNCTION_GRAPH_TRACER option only if > DYNAMIC_FTRACE_WITH_REGS is enabled. > > +config CC_HAS_BRANCH_PROT_NONE > + # GCC 9 or later, clang 8 or later > + def_bool $(cc-option,-mbranch-protection=none) > + > config CC_HAS_BRANCH_PROT_PAC_RET > # GCC 9 or later, clang 8 or later > def_bool $(cc-option,-mbranch-protection=pac-ret+leaf) > diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile > index f15f92ba53e6..370fca6663c8 100644 > --- a/arch/arm64/Makefile > +++ b/arch/arm64/Makefile > @@ -65,6 +65,10 @@ stack_protector_prepare: prepare0 > include/generated/asm-offsets.h)) > endif > > +# Ensure that if the compiler supports branch protection we default it > +# off, this will be overridden if we are using branch protection. > +branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_NONE) := -mbranch-protection=none > + > ifeq ($(CONFIG_ARM64_PTR_AUTH),y) > branch-prot-flags-$(CONFIG_CC_HAS_SIGN_RETURN_ADDRESS) := -msign-return-address=all > branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET) := -mbranch-protection=pac-ret+leaf > @@ -73,9 +77,10 @@ branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET) := -mbranch-protection=pa > # we pass it only to the assembler. This option is utilized only in case of non > # integrated assemblers. > branch-prot-flags-$(CONFIG_AS_HAS_PAC) += -Wa,-march=armv8.3-a > -KBUILD_CFLAGS += $(branch-prot-flags-y) > endif > > +KBUILD_CFLAGS += $(branch-prot-flags-y) > + > ifeq ($(CONFIG_CPU_BIG_ENDIAN), y) > KBUILD_CPPFLAGS += -mbig-endian > CHECKFLAGS += -D__AARCH64EB__ > -- > 2.20.1 > -- Kees Cook