[PATCH 7/8] riscv: add DEBUG_WX support

From: Zong Li <zong.li@sifive.com>
To: paul.walmsley@sifive.com, palmer@dabbelt.com,
	aou@eecs.berkeley.edu, linux-riscv@lists.infradead.org,
	linux-kernel@vger.kernel.org
Cc: Zong Li <zong.li@sifive.com>
Subject: [PATCH 7/8] riscv: add DEBUG_WX support
Date: Mon, 17 Feb 2020 16:32:22 +0800	[thread overview]
Message-ID: <20200217083223.2011-8-zong.li@sifive.com> (raw)
In-Reply-To: <20200217083223.2011-1-zong.li@sifive.com>

Support DEBUG_WX to check whether there are mapping with write and
execute permission at the same time.

Signed-off-by: Zong Li <zong.li@sifive.com>
---
 arch/riscv/Kconfig.debug        | 30 ++++++++++++++++++++++++++++++
 arch/riscv/include/asm/ptdump.h |  6 ++++++
 arch/riscv/mm/init.c            |  2 ++
 3 files changed, 38 insertions(+)

diff --git a/arch/riscv/Kconfig.debug b/arch/riscv/Kconfig.debug
index e69de29bb2d1..2bcd88e75626 100644
--- a/arch/riscv/Kconfig.debug
+++ b/arch/riscv/Kconfig.debug
@@ -0,0 +1,30 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+config DEBUG_WX
+	bool "Warn on W+X mappings at boot"
+	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:
+
+	    riscv/mm: Checked W+X mappings: passed, no W+X pages found.
+
+	  or like this, if the check failed:
+
+	    riscv/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".
diff --git a/arch/riscv/include/asm/ptdump.h b/arch/riscv/include/asm/ptdump.h
index e29af7191909..eb2a1cc5f22c 100644
--- a/arch/riscv/include/asm/ptdump.h
+++ b/arch/riscv/include/asm/ptdump.h
@@ -8,4 +8,10 @@
 
 void ptdump_check_wx(void);
 
+#ifdef CONFIG_DEBUG_WX
+#define debug_checkwx() ptdump_check_wx()
+#else
+#define debug_checkwx() do { } while (0)
+#endif
+
 #endif /* _ASM_RISCV_PTDUMP_H */
diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 09fa643e079c..a05d76e5fefe 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -509,6 +509,8 @@ void mark_rodata_ro(void)
 	set_memory_ro(rodata_start, (data_start - rodata_start) >> PAGE_SHIFT);
 	set_memory_nx(rodata_start, (data_start - rodata_start) >> PAGE_SHIFT);
 	set_memory_nx(data_start, (max_low - data_start) >> PAGE_SHIFT);
+
+	debug_checkwx();
 }
 #endif
 
-- 
2.25.0

