proc read mm's {arg,env}_{start,end} with mmap semaphore taken.

Only functions doing more than one read are modified.  Consumeres
happened to deal with possibly changing data, but it does not seem like
a good thing to rely on.

Signed-off-by: Mateusz Guzik <mguzik@redhat.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Jarod Wilson <jarod@redhat.com>
Cc: Jan Stancek <jstancek@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Anshuman Khandual <anshuman.linux@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 4 deletions

diff --git a/mm/util.c b/mm/util.c
index 6d1f9200f74e..c108a6542d05 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -476,17 +476,25 @@ int get_cmdline(struct task_struct *task, char *buffer, int buflen)
         int res = 0;
         unsigned int len;
         struct mm_struct *mm = get_task_mm(task);
+        unsigned long arg_start, arg_end, env_start, env_end;
         if (!mm)
                 goto out;
         if (!mm->arg_end)
                 goto out_mm;    /* Shh! No looking before we're done */
 
-        len = mm->arg_end - mm->arg_start;
+        down_read(&mm->mmap_sem);
+        arg_start = mm->arg_start;
+        arg_end = mm->arg_end;
+        env_start = mm->env_start;
+        env_end = mm->env_end;
+        up_read(&mm->mmap_sem);
+
+        len = arg_end - arg_start;
 
         if (len > buflen)
                 len = buflen;
 
-        res = access_process_vm(task, mm->arg_start, buffer, len, 0);
+        res = access_process_vm(task, arg_start, buffer, len, 0);
 
         /*
          * If the nul at the end of args has been overwritten, then
@@ -497,10 +505,10 @@ int get_cmdline(struct task_struct *task, char *buffer, int buflen)
                 if (len < res) {
                         res = len;
                 } else {
-                        len = mm->env_end - mm->env_start;
+                        len = env_end - env_start;
                         if (len > buflen - res)
                                 len = buflen - res;
-                        res += access_process_vm(task, mm->env_start,
+                        res += access_process_vm(task, env_start,
                                                  buffer+res, len, 0);
                         res = strnlen(buffer, res);
                 }