proc: avoid information leaks to non-privileged processes

By using the same test as is used for /proc/pid/maps and /proc/pid/smaps,
only allow processes that can ptrace() a given process to see information
that might be used to bypass address space layout randomization (ASLR).
These include eip, esp, wchan, and start_stack in /proc/pid/stat as well
as the non-symbolic output from /proc/pid/wchan.

ASLR can be bypassed by sampling eip as shown by the proof-of-concept
code at http://code.google.com/p/fuzzyaslr/ As part of a presentation
(http://www.cr0.org/paper/to-jt-linux-alsr-leak.pdf) esp and wchan were
also noted as possibly usable information leaks as well.  The
start_stack address also leaks potentially useful information.

Cc: Stable Team <stable@kernel.org>
Signed-off-by: Jake Edge <jake@lwn.net>
Acked-by: Arjan van de Ven <arjan@linux.intel.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/array.c

@@ -80,6 +80,7 @@
 #include <linux/delayacct.h>
 #include <linux/seq_file.h>
 #include <linux/pid_namespace.h>
+#include <linux/ptrace.h>
 #include <linux/tracehook.h>
 
 #include <asm/pgtable.h>
@@ -352,6 +353,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 	char state;
 	pid_t ppid = 0, pgid = -1, sid = -1;
 	int num_threads = 0;
+	int permitted;
 	struct mm_struct *mm;
 	unsigned long long start_time;
 	unsigned long cmin_flt = 0, cmaj_flt = 0;
@@ -364,12 +366,15 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 
 	state = *get_task_state(task);
 	vsize = eip = esp = 0;
+	permitted = ptrace_may_access(task, PTRACE_MODE_READ);
 	mm = get_task_mm(task);
 	if (mm) {
 		vsize = task_vsize(mm);
+		if (permitted) {
 			eip = KSTK_EIP(task);
 			esp = KSTK_ESP(task);
 		}
+	}
 
 	get_task_comm(tcomm, task);
 
@@ -424,7 +429,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 		unlock_task_sighand(task, &flags);
 	}
 
-	if (!whole || num_threads < 2)
+	if (permitted && (!whole || num_threads < 2))
 		wchan = get_wchan(task);
 	if (!whole) {
 		min_flt = task->min_flt;
@@ -476,7 +481,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 		rsslim,
 		mm ? mm->start_code : 0,
 		mm ? mm->end_code : 0,
-		mm ? mm->start_stack : 0,
+		(permitted && mm) ? mm->start_stack : 0,
 		esp,
 		eip,
 		/* The signal information here is obsolete.

fs/proc/base.c

@@ -322,6 +322,9 @@ static int proc_pid_wchan(struct task_struct *task, char *buffer)
 	wchan = get_wchan(task);
 
 	if (lookup_symbol_name(wchan, symname) < 0)
+		if (!ptrace_may_access(task, PTRACE_MODE_READ))
+			return 0;
+		else
 			return sprintf(buffer, "%lu", wchan);
 	else
 		return sprintf(buffer, "%s", symname);