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[PATCH] MIPS: initial stack protector support

To: linux-mips@linux-mips.org
Subject: [PATCH] MIPS: initial stack protector support
From: "Gregory Fong" <gregory.0xf0@gmail.com>
Date: Wed, 12 Jun 2013 11:08:54 -0700
Cc: "Filippo Arcidiacono" <filippo.arcidiacono@st.com>, "Carmelo Amoroso" <carmelo.amoroso@st.com>, "Gregory Fong" <gregory.0xf0@gmail.com>
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Implements basic stack protector support based on ARM version in
c743f38013aeff58ef6252601e397b5ba281c633 , with Kconfig option,
constant canary value set at boot time, and script to check if
compiler actually supports stack protector.

Tested by creating a kernel module that writes past end of char[].

Signed-off-by: Gregory Fong <gregory.0xf0@gmail.com>
---
 arch/mips/Kconfig                      |   13 +++++++++++
 arch/mips/Makefile                     |    4 ++++
 arch/mips/include/asm/stackprotector.h |   40 ++++++++++++++++++++++++++++++++
 arch/mips/kernel/process.c             |    6 +++++
 4 files changed, 63 insertions(+)
 create mode 100644 arch/mips/include/asm/stackprotector.h

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 7a58ab9..a241588 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -2381,6 +2381,19 @@ config SECCOMP
 
          If unsure, say Y. Only embedded should say N here.
 
+config CC_STACKPROTECTOR
+       bool "Enable -fstack-protector buffer overflow detection (EXPERIMENTAL)"
+       help
+         This option turns on the -fstack-protector GCC feature. This
+         feature puts, at the beginning of functions, a canary value on
+         the stack just before the return address, and validates
+         the value just before actually returning.  Stack based buffer
+         overflows (that need to overwrite this return address) now also
+         overwrite the canary, which gets detected and the attack is then
+         neutralized via a kernel panic.
+
+         This feature requires gcc version 4.2 or above.
+
 config USE_OF
        bool
        select OF
diff --git a/arch/mips/Makefile b/arch/mips/Makefile
index dd58a04..37f9ef3 100644
--- a/arch/mips/Makefile
+++ b/arch/mips/Makefile
@@ -227,6 +227,10 @@ KBUILD_CPPFLAGS += -DDATAOFFSET=$(if 
$(dataoffset-y),$(dataoffset-y),0)
 
 LDFLAGS                        += -m $(ld-emul)
 
+ifdef CONFIG_CC_STACKPROTECTOR
+  KBUILD_CFLAGS += -fstack-protector
+endif
+
 ifdef CONFIG_MIPS
 CHECKFLAGS += $(shell $(CC) $(KBUILD_CFLAGS) -dM -E -x c /dev/null | \
        egrep -vw '__GNUC_(|MINOR_|PATCHLEVEL_)_' | \
diff --git a/arch/mips/include/asm/stackprotector.h 
b/arch/mips/include/asm/stackprotector.h
new file mode 100644
index 0000000..eb9b103
--- /dev/null
+++ b/arch/mips/include/asm/stackprotector.h
@@ -0,0 +1,40 @@
+/*
+ * GCC stack protector support.
+ *
+ * (This is directly adopted from the ARM implementation)
+ *
+ * Stack protector works by putting predefined pattern at the start of
+ * the stack frame and verifying that it hasn't been overwritten when
+ * returning from the function.  The pattern is called stack canary
+ * and gcc expects it to be defined by a global variable called
+ * "__stack_chk_guard" on MIPS.  This unfortunately means that on SMP
+ * we cannot have a different canary value per task.
+ */
+
+#ifndef _ASM_STACKPROTECTOR_H
+#define _ASM_STACKPROTECTOR_H 1
+
+#include <linux/random.h>
+#include <linux/version.h>
+
+extern unsigned long __stack_chk_guard;
+
+/*
+ * Initialize the stackprotector canary value.
+ *
+ * NOTE: this must only be called from functions that never return,
+ * and it must always be inlined.
+ */
+static __always_inline void boot_init_stack_canary(void)
+{
+       unsigned long canary;
+
+       /* Try to get a semi random initial value. */
+       get_random_bytes(&canary, sizeof(canary));
+       canary ^= LINUX_VERSION_CODE;
+
+       current->stack_canary = canary;
+       __stack_chk_guard = current->stack_canary;
+}
+
+#endif /* _ASM_STACKPROTECTOR_H */
diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
index c6a041d..7d62894 100644
--- a/arch/mips/kernel/process.c
+++ b/arch/mips/kernel/process.c
@@ -201,6 +201,12 @@ int dump_task_fpu(struct task_struct *t, elf_fpregset_t 
*fpr)
        return 1;
 }
 
+#ifdef CONFIG_CC_STACKPROTECTOR
+#include <linux/stackprotector.h>
+unsigned long __stack_chk_guard __read_mostly;
+EXPORT_SYMBOL(__stack_chk_guard);
+#endif
+
 /*
  *
  */
-- 
1.7.9.5



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