linux-kernel - [PATCH v6 23/29] x86/build: Enforce an empty .got.plt section

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Message-Id: <20200821194310.3089815-24-keescook@chromium.org>
Date:   Fri, 21 Aug 2020 12:43:04 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Peter Collingbourne <pcc@...gle.com>,
        James Morse <james.morse@....com>,
        Borislav Petkov <bp@...e.de>, Ingo Molnar <mingo@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>, x86@...nel.org,
        clang-built-linux@...glegroups.com, linux-arch@...r.kernel.org,
        linux-efi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v6 23/29] x86/build: Enforce an empty .got.plt section

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 INFO. This is more sensitive than just blindly
discarding the section.

Signed-off-by: Kees Cook <keescook@...omium.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..4b1b936a6e7d 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 (INFO) : { *(.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