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>

4 files changed, 35 insertions, 34 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index d9512bd03e6c..d4548dd49b02 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -198,26 +198,6 @@ static int proc_root_link(struct dentry *dentry, struct path *path)
         return result;
 }
 
-static struct mm_struct *mm_access(struct task_struct *task, unsigned int mode)
-{
-        struct mm_struct *mm;
-        int err;
-
-        err =  mutex_lock_killable(&task->signal->cred_guard_mutex);
-        if (err)
-                return ERR_PTR(err);
-
-        mm = get_task_mm(task);
-        if (mm && mm != current->mm &&
-                        !ptrace_may_access(task, mode)) {
-                mmput(mm);
-                mm = ERR_PTR(-EACCES);
-        }
-        mutex_unlock(&task->signal->cred_guard_mutex);
-
-        return mm;
-}
-
 struct mm_struct *mm_for_maps(struct task_struct *task)
 {
         return mm_access(task, PTRACE_MODE_READ);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2234985a5e65..7d379a6bfd88 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2259,6 +2259,12 @@ static inline void mmdrop(struct mm_struct * mm)
 extern void mmput(struct mm_struct *);
 /* Grab a reference to a task's mm, if it is not already going away */
 extern struct mm_struct *get_task_mm(struct task_struct *task);
+/*
+ * Grab a reference to a task's mm, if it is not already going away
+ * and ptrace_may_access with the mode parameter passed to it
+ * succeeds.
+ */
+extern struct mm_struct *mm_access(struct task_struct *task, unsigned int mode);
 /* Remove the current tasks stale references to the old mm_struct */
 extern void mm_release(struct task_struct *, struct mm_struct *);
 /* Allocate a new mm structure and copy contents from tsk->mm */
diff --git a/kernel/fork.c b/kernel/fork.c
index 051f090d40c1..1b2ef3c23ae4 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -647,6 +647,26 @@ struct mm_struct *get_task_mm(struct task_struct *task)
 }
 EXPORT_SYMBOL_GPL(get_task_mm);
 
+struct mm_struct *mm_access(struct task_struct *task, unsigned int mode)
+{
+        struct mm_struct *mm;
+        int err;
+
+        err =  mutex_lock_killable(&task->signal->cred_guard_mutex);
+        if (err)
+                return ERR_PTR(err);
+
+        mm = get_task_mm(task);
+        if (mm && mm != current->mm &&
+                        !ptrace_may_access(task, mode)) {
+                mmput(mm);
+                mm = ERR_PTR(-EACCES);
+        }
+        mutex_unlock(&task->signal->cred_guard_mutex);
+
+        return mm;
+}
+
 /* Please note the differences between mmput and mm_release.
  * mmput is called whenever we stop holding onto a mm_struct,
  * error success whatever.
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,