linux-kernel - [PATCH 1/4] mm: add DEBUG

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Date:   Tue, 21 Apr 2020 16:17:12 +0800
From:   Zong Li <zong.li@...ive.com>
To:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, paul.walmsley@...ive.com,
        palmer@...belt.com, linux-riscv@...ts.infradead.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        hpa@...or.com, catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org
Cc:     Zong Li <zong.li@...ive.com>
Subject: [PATCH 1/4] mm: add DEBUG_WX support

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

Signed-off-by: Zong Li <zong.li@...ive.com>
Suggested-by: Palmer Dabbelt <palmer@...belt.com>
---
 mm/Kconfig.debug | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug
index 0271b22e063f..077458ad968d 100644
--- a/mm/Kconfig.debug
+++ b/mm/Kconfig.debug
@@ -118,6 +118,39 @@ 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
+	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.
+	  This check also includes UXN, which should be set on all kernel
+	  mappings.
+
+	  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
 
-- 
2.26.1