linux-kernel - [PATCH] x86/mm: Disable W^X detection and enforcement on 32-bit

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Date:   Fri, 23 Sep 2022 15:17:30 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     linux-kernel@...r.kernel.org
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Ard Biesheuvel <ardb@...nel.org>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, linux-efi@...r.kernel.org,
        "H. Peter Anvin" <hpa@...or.com>, Kees Cook <keescook@...omium.org>
Subject: [PATCH] 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 <linux@...ck-us.net>
Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Ard Biesheuvel <ardb@...nel.org>
Cc: Darren Hart <dvhart@...radead.org>
Cc: Andy Shevchenko <andy@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: x86@...nel.org
Cc: linux-efi@...r.kernel.org
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Kees Cook <keescook@...omium.org>
Link: https://lore.kernel.org/all/CAMj1kXHcF_iK_g0OZSkSv56Wmr=eQGQwNstcNjLEfS=mm7a06w@mail.gmail.com/
---
 arch/x86/mm/pat/set_memory.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index 20b1e24baa85..efe882c753ca 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/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;
-- 
2.34.1