x86/pti: Disallow global kernel text with RANDSTRUCT

commit 26d35ca6c3776784f8156e1d6f80cc60d9a2a915

RANDSTRUCT derives its hardening benefits from the attacker's lack of
knowledge about the layout of kernel data structures.  Keep the kernel
image non-global in cases where RANDSTRUCT is in use to help keep the
layout a secret.

Fixes: 8c06c77 (x86/pti: Leave kernel text global for !PCID)
Reported-by: Kees Cook <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: Ingo Molnar <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: [email protected]
Cc: Linus Torvalds <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Nadav Amit <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Arjan van de Ven <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

Author: hansendc

arch/x86/mm/pti.c

@@ -421,6 +421,16 @@ static inline bool pti_kernel_image_global_ok(void)
 	if (boot_cpu_has(X86_FEATURE_K8))
 		return false;
 
+	/*
+	 * RANDSTRUCT derives its hardening benefits from the
+	 * attacker's lack of knowledge about the layout of kernel
+	 * data structures.  Keep the kernel image non-global in
+	 * cases where RANDSTRUCT is in use to help keep the layout a
+	 * secret.
+	 */
+	if (IS_ENABLED(CONFIG_GCC_PLUGIN_RANDSTRUCT))
+		return false;
+
 	return true;
 }