mm: add DEBUG_WX support

Patch series "Extract DEBUG_WX to shared use".

Some architectures support DEBUG_WX function, it's verbatim from each
others, so extract to mm/Kconfig.debug for shared use.

PPC and ARM ports don't support generic page dumper yet, so we only
refine x86 and arm64 port in this patch series.

For RISC-V port, the DEBUG_WX support depends on other patches which
be merged already:
  - RISC-V page table dumper
  - Support strict kernel memory permissions for security

This patch (of 4):

Some architectures support DEBUG_WX function, it's verbatim from each
others.  Extract to mm/Kconfig.debug for shared use.

[[email protected]: reword text, per Will Deacon & Zong Li]
  Link: http://lkml.kernel.org/r/20200427194245.oxRJKj3fn%[email protected]
[[email protected]: remove the specific name of arm64]
  Link: http://lkml.kernel.org/r/3a6a92ecedc54e1d0fc941398e63d504c2cd5611.1589178399.git.zong.li@sifive.com
[[email protected]: add MMU dependency for DEBUG_WX]
  Link: http://lkml.kernel.org/r/4a674ac7863ff39ca91847b10e51209771f99416.1589178399.git.zong.li@sifive.com
Suggested-by: Palmer Dabbelt <[email protected]>
Signed-off-by: Zong Li <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Cc: Paul Walmsley <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Will Deacon <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/23980cd0f0e5d79e24a92169116407c75bcc650d.1587455584.git.zong.li@sifive.com
Signed-off-by: Linus Torvalds <[email protected]>

mm/Kconfig.debug

@@ -118,6 +118,38 @@ config DEBUG_RODATA_TEST
     ---help---
       This option enables a testcase for the setting rodata read-only.
 
+config ARCH_HAS_DEBUG_WX
+	bool
+
+config DEBUG_WX
+	bool "Warn on W+X mappings at boot"
+	depends on ARCH_HAS_DEBUG_WX
+	depends on MMU
+	select PTDUMP_CORE
+	help
+	  Generate a warning if any W+X mappings are found at boot.
+
+	  This is useful for discovering cases where the kernel is leaving W+X
+	  mappings after applying NX, as such mappings are a security risk.
+
+	  Look for a message in dmesg output like this:
+
+	    <arch>/mm: Checked W+X mappings: passed, no W+X pages found.
+
+	  or like this, if the check failed:
+
+	    <arch>/mm: Checked W+X mappings: failed, <N> W+X pages found.
+
+	  Note that even if the check fails, your kernel is possibly
+	  still fine, as W+X mappings are not a security hole in
+	  themselves, what they do is that they make the exploitation
+	  of other unfixed kernel bugs easier.
+
+	  There is no runtime or memory usage effect of this option
+	  once the kernel has booted up - it's a one time check.
+
+	  If in doubt, say "Y".
+
 config GENERIC_PTDUMP
 	bool