[128/131] mm: add DEBUG_WX support

Commit Message

Andrew Morton June 3, 2020, 11:03 p.m. UTC

From: Zong Li <[e-mail]>
Subject: 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.

[[e-mail]: reword text, per Will Deacon & Zong Li]
  Link: http://lkml.kernel.org/r/[e-mail]
[[e-mail]: remove the specific name of arm64]
  Link: http://lkml.kernel.org/r/[e-mail]
[[e-mail]: add MMU dependency for DEBUG_WX]
  Link: http://lkml.kernel.org/r/[e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Zong Li <[e-mail]>
Suggested-by: Palmer Dabbelt <[e-mail]>
Cc: Paul Walmsley <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: "H. Peter Anvin" <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: Will Deacon <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/Kconfig.debug |   32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

Patch

--- a/mm/Kconfig.debug~mm-add-debug_wx-support
+++ a/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