Fix race in process_vm_rw_core

This fixes the race in process_vm_core found by Oleg (see

  http://article.gmane.org/gmane.linux.kernel/1235667/

for details).

This has been updated since I last sent it as the creation of the new
mm_access() function did almost exactly the same thing as parts of the
previous version of this patch did.

In order to use mm_access() even when /proc isn't enabled, we move it to
kernel/fork.c where other related process mm access functions already
are.

Signed-off-by: Chris Yeoh <yeohc@au1.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 14 deletions

diff --git a/mm/process_vm_access.c b/mm/process_vm_access.c
index e920aa3ce104..c20ff48994c2 100644
--- a/mm/process_vm_access.c
+++ b/mm/process_vm_access.c
@@ -298,23 +298,18 @@ static ssize_t process_vm_rw_core(pid_t pid, const struct iovec *lvec,
                 goto free_proc_pages;
         }
 
-        task_lock(task);
-        if (__ptrace_may_access(task, PTRACE_MODE_ATTACH)) {
-                task_unlock(task);
-                rc = -EPERM;
-                goto put_task_struct;
-        }
-        mm = task->mm;
-
-        if (!mm || (task->flags & PF_KTHREAD)) {
-                task_unlock(task);
-                rc = -EINVAL;
+        mm = mm_access(task, PTRACE_MODE_ATTACH);
+        if (!mm || IS_ERR(mm)) {
+                rc = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH;
+                /*
+                 * Explicitly map EACCES to EPERM as EPERM is a more a
+                 * appropriate error code for process_vw_readv/writev
+                 */
+                if (rc == -EACCES)
+                        rc = -EPERM;
                 goto put_task_struct;
         }
 
-        atomic_inc(&mm->mm_users);
-        task_unlock(task);
-
         for (i = 0; i < riovcnt && iov_l_curr_idx < liovcnt; i++) {
                 rc = process_vm_rw_single_vec(
                         (unsigned long)rvec[i].iov_base, rvec[i].iov_len,