proc: make check_mem_permission() return an mm_struct on success

This change allows us to take advantage of access_remote_vm(), which in turn
eliminates a security issue with the mem_write() implementation.

The previous implementation of mem_write() was insecure since the target task
could exec a setuid-root binary between the permission check and the actual
write.  Holding a reference to the target mm_struct eliminates this
vulnerability.

Signed-off-by: Stephen Wilson <wilsons@start.ca>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 34 insertions, 24 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 013f116b3223..e34c3c33b2de 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -191,14 +191,20 @@ static int proc_root_link(struct inode *inode, struct path *path)
         return result;
 }
 
-static int __check_mem_permission(struct task_struct *task)
+static struct mm_struct *__check_mem_permission(struct task_struct *task)
 {
+        struct mm_struct *mm;
+
+        mm = get_task_mm(task);
+        if (!mm)
+                return ERR_PTR(-EINVAL);
+
         /*
          * A task can always look at itself, in case it chooses
          * to use system calls instead of load instructions.
          */
         if (task == current)
-                return 0;
+                return mm;
 
         /*
          * If current is actively ptrace'ing, and would also be
@@ -210,20 +216,23 @@ static int __check_mem_permission(struct task_struct *task)
                 match = (tracehook_tracer_task(task) == current);
                 rcu_read_unlock();
                 if (match && ptrace_may_access(task, PTRACE_MODE_ATTACH))
-                        return 0;
+                        return mm;
         }
 
         /*
          * Noone else is allowed.
          */
-        return -EPERM;
+        mmput(mm);
+        return ERR_PTR(-EPERM);
 }
 
 /*
- * Return zero if current may access user memory in @task, -error if not.
+ * If current may access user memory in @task return a reference to the
+ * corresponding mm, otherwise ERR_PTR.
  */
-static int check_mem_permission(struct task_struct *task)
+static struct mm_struct *check_mem_permission(struct task_struct *task)
 {
+        struct mm_struct *mm;
         int err;
 
         /*
@@ -232,12 +241,12 @@ static int check_mem_permission(struct task_struct *task)
          */
         err = mutex_lock_killable(&task->signal->cred_guard_mutex);
         if (err)
-                return err;
+                return ERR_PTR(err);
 
-        err = __check_mem_permission(task);
+        mm = __check_mem_permission(task);
         mutex_unlock(&task->signal->cred_guard_mutex);
 
-        return err;
+        return mm;
 }
 
 struct mm_struct *mm_for_maps(struct task_struct *task)
@@ -795,18 +804,14 @@ static ssize_t mem_read(struct file * file, char __user * buf,
         if (!task)
                 goto out_no_task;
 
-        if (check_mem_permission(task))
-                goto out;
-
         ret = -ENOMEM;
         page = (char *)__get_free_page(GFP_TEMPORARY);
         if (!page)
                 goto out;
 
-        ret = 0;
- 
-        mm = get_task_mm(task);
-        if (!mm)
+        mm = check_mem_permission(task);
+        ret = PTR_ERR(mm);
+        if (IS_ERR(mm))
                 goto out_free;
 
         ret = -EIO;
@@ -820,8 +825,8 @@ static ssize_t mem_read(struct file * file, char __user * buf,
                 int this_len, retval;
 
                 this_len = (count > PAGE_SIZE) ? PAGE_SIZE : count;
-                retval = access_process_vm(task, src, page, this_len, 0);
-                if (!retval || check_mem_permission(task)) {
+                retval = access_remote_vm(mm, src, page, this_len, 0);
+                if (!retval) {
                         if (!ret)
                                 ret = -EIO;
                         break;
@@ -860,22 +865,25 @@ static ssize_t mem_write(struct file * file, const char __user *buf,
         char *page;
         struct task_struct *task = get_proc_task(file->f_path.dentry->d_inode);
         unsigned long dst = *ppos;
+        struct mm_struct *mm;
 
         copied = -ESRCH;
         if (!task)
                 goto out_no_task;
 
-        if (check_mem_permission(task))
-                goto out;
+        mm = check_mem_permission(task);
+        copied = PTR_ERR(mm);
+        if (IS_ERR(mm))
+                goto out_task;
 
         copied = -EIO;
         if (file->private_data != (void *)((long)current->self_exec_id))
-                goto out;
+                goto out_mm;
 
         copied = -ENOMEM;
         page = (char *)__get_free_page(GFP_TEMPORARY);
         if (!page)
-                goto out;
+                goto out_mm;
 
         copied = 0;
         while (count > 0) {
@@ -886,7 +894,7 @@ static ssize_t mem_write(struct file * file, const char __user *buf,
                         copied = -EFAULT;
                         break;
                 }
-                retval = access_process_vm(task, dst, page, this_len, 1);
+                retval = access_remote_vm(mm, dst, page, this_len, 1);
                 if (!retval) {
                         if (!copied)
                                 copied = -EIO;
@@ -899,7 +907,9 @@ static ssize_t mem_write(struct file * file, const char __user *buf,
         }
         *ppos = dst;
         free_page((unsigned long) page);
-out:
+out_mm:
+        mmput(mm);
+out_task:
         put_task_struct(task);
 out_no_task:
         return copied;