x86-64: Fill unused parts of the vsyscall page with 0xcc

Jumping to 0x00 might do something depending on the following
bytes. Jumping to 0xcc is a trap. So fill the unused parts of
the vsyscall page with 0xcc to make it useless for exploits to
jump there.

Signed-off-by: Andy Lutomirski <[e-mail]>
Cc: Jesper Juhl <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Arjan van de Ven <[e-mail]>
Cc: Jan Beulich <[e-mail]>
Cc: richard -rw- weinberger <[e-mail]>
Cc: Mikael Pettersson <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Brian Gerst <[e-mail]>
Cc: Louis Rilling <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/kernel/vmlinux.lds.S

···
 	__vsyscall_0 = .;
 
 	. = VSYSCALL_ADDR;
-	.vsyscall_0 : AT(VLOAD(.vsyscall_0)) {
+	.vsyscall : AT(VLOAD(.vsyscall)) {
 		*(.vsyscall_0)
-	} :user
 
-	. = ALIGN(L1_CACHE_BYTES);
-	.vsyscall_fn : AT(VLOAD(.vsyscall_fn)) {
+		. = ALIGN(L1_CACHE_BYTES);
 		*(.vsyscall_fn)
-	}
 
-	.vsyscall_1 ADDR(.vsyscall_0) + 1024: AT(VLOAD(.vsyscall_1)) {
+		. = 1024;
 		*(.vsyscall_1)
-	}
-	.vsyscall_2 ADDR(.vsyscall_0) + 2048: AT(VLOAD(.vsyscall_2)) {
-		*(.vsyscall_2)
-	}
 
+		. = 2048;
+		*(.vsyscall_2)
+
+		. = 4096;  /* Pad the whole page. */
+	} :user =0xcc
 	. = ALIGN(__vsyscall_0 + PAGE_SIZE, PAGE_SIZE);
 
 #undef VSYSCALL_ADDR