Re: [PATCH v2 1/4] syscalls: Restore address limit after a syscall

On Thu, Mar 09, 2017 at 12:09:55PM +0000, Mark Rutland wrote:

Hi,

On Wed, Mar 08, 2017 at 05:24:53PM -0800, Thomas Garnier wrote:

quoted

This patch ensures a syscall does not return to user-mode with a kernel
address limit. If that happened, a process can corrupt kernel-mode
memory and elevate privileges.

For example, it would mitigation this bug:

- https://bugs.chromium.org/p/project-zero/issues/detail?id=990

If the CONFIG_BUG_ON_DATA_CORRUPTION option is enabled, an incorrect
state will result in a BUG_ON.

The CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE option is also
added so each architecture can optimize this change.

quoted

+#ifndef CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE
+static inline bool has_user_ds(void) {
+	bool ret = segment_eq(get_fs(), USER_DS);
+	// Prevent re-ordering the call
+	barrier();

What ordering are we trying to ensure, that isn't otherwise given?

We expect get_fs() and set_fs() to be ordered w.r.t. each other and
w.r.t. uaccess uses, or we'd need barriers all over the place.

Given that, I can't see why we need a barrier here. So this needs a
better comment, at least.

quoted

+	return ret;
+}
+#else
+static inline bool has_user_ds(void) {
+	return false;
+}
+#endif

It would be simpler to wrap the call entirely, e.g. have:

#ifdef CONFIG_WHATEVER
static inline void verify_pre_usermode_state(void)
{
	if (segment_eq(get_fs(), USER_DS))
		__verify_pre_usermode_state();
}
#else
static inline void verify_pre_usermode_state(void) { }
#endif

That's utterly pointless - you've missed a detail.

quoted

@@ -199,7 +215,10 @@ extern struct trace_event_functions exit_syscall_print_funcs;
 	asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__));	\
 	asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__))	\
 	{								\
+		bool user_caller = has_user_ds();			\
 		long ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__));	\
+		if (user_caller)					\
+			verify_pre_usermode_state();			\

... then we can unconditionally use verify_pre_usermode_state() here ... 

Look at this closely.  has_user_ds() is called _before_ the syscall code
is invoked.  It's checking what conditions the syscall was entered from.
If the syscall was entered with the user segment selected, then we run
a check on the system state _after_ the syscall code has returned.

Putting both after the syscall code has returned is completely pointless -
it turns it into this code:

	if (segment_eq(get_fs(), USER_DS))
		if (CHECK_DATA_CORRUPTION(!segment_eq(get_fs(), USER_DS),
					  "incorrect get_fs() on user-mode return"))
			set_fs(USER_DS);

which is obviously bogus (it'll never fire.)

-- 
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