MIPS: initial stack protector support

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>
Cc: linux-mips@linux-mips.org
Cc: Filippo Arcidiacono <filippo.arcidiacono@st.com>
Cc: Carmelo Amoroso <carmelo.amoroso@st.com>
Patchwork: https://patchwork.linux-mips.org/patch/5448/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 69bf310..6dbeb27 100644 (file)
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -2367,6 +2367,19 @@ 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 (file)
--- 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 (file)
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 (file)
--- 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
+
 /*
  *
  */