[PATCH 1/4] SROP mitigation: Architecture independent code for signal canaries

[Erik Bosman <erik@minemu.org>]

Architecture independent code for signal canaries

Add support for canary values in user-space signal frames.  These canaries
function much like stack canaries/cookies, making it harder for an attacker to
fake a call to {rt_,}sigreturn()

This patch deals with architecture independent changes needed to support
these canaries.


These patches are meant to make Sigreturn Oriented Programming (SROP) a much
less attractive exploitation path.  In Sigreturn Oriented Programming, an
attacker causes a user-space program to call the sigreturn system call in order
to get complete control control over the entire userspace context in one go.

( see: http://www.cs.vu.nl/~herbertb/papers/srop_sp14.pdf )

While mitigating SROP will probably not stop determined attackers from
exploiting a program, as there's always the much more well-known Return
Oriented Programming, we still think SROP's relative ease warrants mitigation,
especially since the mitigation is so cheap.


Signed-off-by: Erik Bosman <erik@minemu.org>

---
 arch/Kconfig          | 16 ++++++++++++++++
 fs/exec.c             |  8 ++++++++
 include/linux/sched.h |  5 +++++
 3 files changed, 29 insertions(+)

diff --git a/arch/Kconfig b/arch/Kconfig
index 97ff872..8319984 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -399,6 +399,22 @@ config CC_STACKPROTECTOR_STRONG
 
 endchoice
 
+config HAVE_SIGNAL_CANARY
+	bool
+	help
+	  An arch should select this symbol if:
+	  - its struct sigframe contains a canary field
+	  - it has implemented signal canary checking
+
+config SIGNAL_CANARY
+	bool "signal canary"
+	default y
+	depends on HAVE_SIGNAL_CANARY
+	help
+	  Mitigate against a userland exploitation techinque called
+	  sigreturn oriented programming by putting a canary value on a
+	  signal's struct sigframe
+
 config HAVE_CONTEXT_TRACKING
 	bool
 	help
diff --git a/fs/exec.c b/fs/exec.c
index 476f3eb..883f456 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -56,6 +56,7 @@
 #include <linux/pipe_fs_i.h>
 #include <linux/oom.h>
 #include <linux/compat.h>
+#include <linux/random.h>
 
 #include <asm/uaccess.h>
 #include <asm/mmu_context.h>
@@ -1105,6 +1106,13 @@ void setup_new_exec(struct linux_binprm * bprm)
 	/* This is the point of no return */
 	current->sas_ss_sp = current->sas_ss_size = 0;
 
+#ifdef CONFIG_SIGNAL_CANARY
+	/* canary value to mitigate the use of sigreturn in (userland) exploits
+	 * get_random_int() should be random enough also for 64bit
+	 */
+	current->signal_canary = (unsigned long)get_random_int();
+#endif
+
 	if (uid_eq(current_euid(), current_uid()) && gid_eq(current_egid(), current_gid()))
 		set_dumpable(current->mm, SUID_DUMP_USER);
 	else
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 25f54c7..cb8b54b 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1364,6 +1364,11 @@ struct task_struct {
 
 	unsigned long sas_ss_sp;
 	size_t sas_ss_size;
+
+#ifdef CONFIG_SIGNAL_CANARY
+	u32 signal_canary; /* sigreturn exploit mitigation */
+#endif
+
 	int (*notifier)(void *priv);
 	void *notifier_data;
 	sigset_t *notifier_mask;
-- 
1.9.1