x86/mm: Disable W^X detection and enforcement on 32-bit

The 32-bit code is in a weird spot.  Some 32-bit builds (non-PAE) do not
even have NX support.  Even PAE builds that support NX have to contend
with things like EFI data and code mixed in the same pages where W+X
is unavoidable.

The folks still running X86_32=y kernels are unlikely to care much about
NX.  That combined with the fundamental inability fix _all_ of the W+X
things means this code had little value on X86_32=y.  Disable the checks.

Reported-by: Guenter Roeck <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Cc: Ard Biesheuvel <[email protected]>
Cc: Darren Hart <[email protected]>
Cc: Andy Shevchenko <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: "H. Peter Anvin" <[email protected]>
Cc: Kees Cook <[email protected]>
Link: https://lore.kernel.org/all/CAMj1kXHcF_iK_g0OZSkSv56Wmr=eQGQwNstcNjLEfS=mm7a06w@mail.gmail.com/

Author: hansendc

arch/x86/mm/pat/set_memory.c

@@ -587,6 +587,14 @@ static inline pgprot_t verify_rwx(pgprot_t old, pgprot_t new, unsigned long star
 {
 	unsigned long end;
 
+	/*
+	 * 32-bit has some unfixable W+X issues, like EFI code
+	 * and writeable data being in the same page.  Disable
+	 * detection and enforcement there.
+	 */
+	if (IS_ENABLED(CONFIG_X86_32))
+		return new;
+
 	/* Only enforce when NX is supported: */
 	if (!(__supported_pte_mask & _PAGE_NX))
 		return new;