[PATCH v5 01/36] x86/boot/compressed: Move .got.plt entries out of the .got section

[Kees Cook]

From: Ard Biesheuvel <ardb at kernel.org>

The .got.plt section contains the part of the GOT which is used by PLT
entries, and which gets updated lazily by the dynamic loader when
function calls are dispatched through those PLT entries.

On fully linked binaries such as the kernel proper or the decompressor,
this never happens, and so in practice, the .got.plt section consists
only of the first 3 magic entries that are meant to point at the _DYNAMIC
section and at the fixup routine in the loader. However, since we don't
use a dynamic loader, those entries are never populated or used.

This means that treating those entries like ordinary GOT entries, and
updating their values based on the actual placement of the executable in
memory is completely pointless, and we can just ignore the .got.plt
section entirely, provided that it has no additional entries beyond
the first 3 ones.

So add an assertion in the linker script to ensure that this assumption
holds, and move the contents out of the [_got, _egot) memory range that
is modified by the GOT fixup routines.

While at it, drop the KEEP(), since it has no effect on the contents
of output sections that are created by the linker itself.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
Signed-off-by: Arvind Sankar <nivedita at alum.mit.edu>
Tested-by: Sedat Dilek <sedat.dilek at gmail.com>
Tested-by: Nick Desaulniers <ndesaulniers at google.com>
Reviewed-by: Kees Cook <keescook at chromium.org>
Acked-by: Arvind Sankar <nivedita at alum.mit.edu>
Link: https://lore.kernel.org/r/20200523120021.34996-2-ardb@kernel.org
Signed-off-by: Kees Cook <keescook at chromium.org>
---
 arch/x86/boot/compressed/vmlinux.lds.S | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
index 8f1025d1f681..b17d218ccdf9 100644
--- a/arch/x86/boot/compressed/vmlinux.lds.S
+++ b/arch/x86/boot/compressed/vmlinux.lds.S
@@ -44,10 +44,13 @@ SECTIONS
 	}
 	.got : {
 		_got = .;
-		KEEP(*(.got.plt))
 		KEEP(*(.got))
 		_egot = .;
 	}
+	.got.plt : {
+		*(.got.plt)
+	}
+
 	.data :	{
 		_data = . ;
 		*(.data)
@@ -77,3 +80,9 @@ SECTIONS
 
 	DISCARDS
 }
+
+#ifdef CONFIG_X86_64
+ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, "Unexpected GOT/PLT entries detected!")
+#else
+ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0xc, "Unexpected GOT/PLT entries detected!")
+#endif
-- 
2.25.1