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>

1 files changed, 9 insertions, 4 deletions

diff --git a/fs/proc/array.c b/fs/proc/array.c
index 7e4877d9dcb5..725a650bbbb8 100644
--- a/fs/proc/array.c
+++ b/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;
@@ -364,11 +366,14 @@ 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);
-                eip = KSTK_EIP(task);
-                esp = KSTK_ESP(task);
+                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.