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From:   Kees Cook <keescook@chromium.org>
To:     Thomas Gleixner <tglx@linutronix.de>, Will Deacon <will@kernel.org>
Cc:     Kees Cook <keescook@chromium.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Peter Collingbourne <pcc@google.com>,
        James Morse <james.morse@arm.com>,
        Borislav Petkov <bp@suse.de>, Ingo Molnar <mingo@redhat.com>,
        Russell King <linux@armlinux.org.uk>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Arnd Bergmann <arnd@arndb.de>, x86@kernel.org,
        clang-built-linux@googlegroups.com, linux-arch@vger.kernel.org,
        linux-efi@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org
Subject: [PATCH v5 29/36] x86/build: Enforce an empty .got.plt section
Date:   Fri, 31 Jul 2020 16:08:13 -0700
Message-Id: <20200731230820.1742553-30-keescook@chromium.org>
In-Reply-To: <20200731230820.1742553-1-keescook@chromium.org>
References: <20200731230820.1742553-1-keescook@chromium.org>
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The .got.plt section should always be zero (or filled only with the
linker-generated lazy dispatch entry). Enforce this with an assert and
mark the section as NOLOAD. This is more sensitive than just blindly
discarding the section.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
 arch/x86/kernel/vmlinux.lds.S | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 0cc035cb15f1..7faffe7414d6 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -414,8 +414,20 @@ SECTIONS
 	ELF_DETAILS
 
 	DISCARDS
-}
 
+	/*
+	 * Make sure that the .got.plt is either completely empty or it
+	 * contains only the lazy dispatch entries.
+	 */
+	.got.plt (NOLOAD) : { *(.got.plt) }
+	ASSERT(SIZEOF(.got.plt) == 0 ||
+#ifdef CONFIG_X86_64
+	       SIZEOF(.got.plt) == 0x18,
+#else
+	       SIZEOF(.got.plt) == 0xc,
+#endif
+	       "Unexpected GOT/PLT entries detected!")
+}
 
 #ifdef CONFIG_X86_32
 /*
-- 
2.25.1