x86/vdso: Mark the vDSO code read-only after init

The vDSO does not need to be writable after __init, so mark it as
__ro_after_init. The result kills the exploit method of writing to the
vDSO from kernel space resulting in userspace executing the modified code,
as shown here to bypass SMEP restrictions: http://itszn.com/blog/?p=21

The memory map (with added vDSO address reporting) shows the vDSO moving
into read-only memory:

Before:
	[    0.143067] vDSO @ ffffffff82004000
	[    0.143551] vDSO @ ffffffff82006000
	---[ High Kernel Mapping ]---
	0xffffffff80000000-0xffffffff81000000      16M                         pmd
	0xffffffff81000000-0xffffffff81800000       8M   ro     PSE     GLB x  pmd
	0xffffffff81800000-0xffffffff819f3000    1996K   ro             GLB x  pte
	0xffffffff819f3000-0xffffffff81a00000      52K   ro                 NX pte
	0xffffffff81a00000-0xffffffff81e00000       4M   ro     PSE     GLB NX pmd
	0xffffffff81e00000-0xffffffff81e05000      20K   ro             GLB NX pte
	0xffffffff81e05000-0xffffffff82000000    2028K   ro                 NX pte
	0xffffffff82000000-0xffffffff8214f000    1340K   RW             GLB NX pte
	0xffffffff8214f000-0xffffffff82281000    1224K   RW                 NX pte
	0xffffffff82281000-0xffffffff82400000    1532K   RW             GLB NX pte
	0xffffffff82400000-0xffffffff83200000      14M   RW     PSE     GLB NX pmd
	0xffffffff83200000-0xffffffffc0000000     974M                         pmd

After:
	[    0.145062] vDSO @ ffffffff81da1000
	[    0.146057] vDSO @ ffffffff81da4000
	---[ High Kernel Mapping ]---
	0xffffffff80000000-0xffffffff81000000      16M                         pmd
	0xffffffff81000000-0xffffffff81800000       8M   ro     PSE     GLB x  pmd
	0xffffffff81800000-0xffffffff819f3000    1996K   ro             GLB x  pte
	0xffffffff819f3000-0xffffffff81a00000      52K   ro                 NX pte
	0xffffffff81a00000-0xffffffff81e00000       4M   ro     PSE     GLB NX pmd
	0xffffffff81e00000-0xffffffff81e0b000      44K   ro             GLB NX pte
	0xffffffff81e0b000-0xffffffff82000000    2004K   ro                 NX pte
	0xffffffff82000000-0xffffffff8214c000    1328K   RW             GLB NX pte
	0xffffffff8214c000-0xffffffff8227e000    1224K   RW                 NX pte
	0xffffffff8227e000-0xffffffff82400000    1544K   RW             GLB NX pte
	0xffffffff82400000-0xffffffff83200000      14M   RW     PSE     GLB NX pmd
	0xffffffff83200000-0xffffffffc0000000     974M                         pmd

Based on work by PaX Team and Brad Spengler.
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Acked-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brad Spengler <spender@grsecurity.net>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: David Brown <david.brown@linaro.org>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Emese Revfy <re.emese@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mathias Krause <minipli@googlemail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: PaX Team <pageexec@freemail.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kernel-hardening@lists.openwall.com
Cc: linux-arch <linux-arch@vger.kernel.org>
Link: http://lkml.kernel.org/r/1455748879-21872-7-git-send-email-keescook@chromium.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/vdso/vdso2c.h

@@ -140,7 +140,7 @@ static void BITSFUNC(go)(void *raw_addr, size_t raw_len,
 	fprintf(outfile, "#include <asm/vdso.h>\n");
 	fprintf(outfile, "\n");
 	fprintf(outfile,
-		"static unsigned char raw_data[%lu] __page_aligned_data = {",
+		"static unsigned char raw_data[%lu] __ro_after_init __aligned(PAGE_SIZE) = {",
 		mapping_size);
 	for (j = 0; j < stripped_len; j++) {
 		if (j % 10 == 0)