fs, proc: fix ABBA deadlock in case of execution attempt of map_files/ entries

map_files/ entries are never supposed to be executed, still curious
minds might try to run them, which leads to the following deadlock

  ======================================================
  [ INFO: possible circular locking dependency detected ]
  3.4.0-rc4-24406-g841e6a6 #121 Not tainted
  -------------------------------------------------------
  bash/1556 is trying to acquire lock:
   (&sb->s_type->i_mutex_key#8){+.+.+.}, at: do_lookup+0x267/0x2b1

  but task is already holding lock:
   (&sig->cred_guard_mutex){+.+.+.}, at: prepare_bprm_creds+0x2d/0x69

  which lock already depends on the new lock.

  the existing dependency chain (in reverse order) is:

  -> #1 (&sig->cred_guard_mutex){+.+.+.}:
         validate_chain+0x444/0x4f4
         __lock_acquire+0x387/0x3f8
         lock_acquire+0x12b/0x158
         __mutex_lock_common+0x56/0x3a9
         mutex_lock_killable_nested+0x40/0x45
         lock_trace+0x24/0x59
         proc_map_files_lookup+0x5a/0x165
         __lookup_hash+0x52/0x73
         do_lookup+0x276/0x2b1
         walk_component+0x3d/0x114
         do_last+0xfc/0x540
         path_openat+0xd3/0x306
         do_filp_open+0x3d/0x89
         do_sys_open+0x74/0x106
         sys_open+0x21/0x23
         tracesys+0xdd/0xe2

  -> #0 (&sb->s_type->i_mutex_key#8){+.+.+.}:
         check_prev_add+0x6a/0x1ef
         validate_chain+0x444/0x4f4
         __lock_acquire+0x387/0x3f8
         lock_acquire+0x12b/0x158
         __mutex_lock_common+0x56/0x3a9
         mutex_lock_nested+0x40/0x45
         do_lookup+0x267/0x2b1
         walk_component+0x3d/0x114
         link_path_walk+0x1f9/0x48f
         path_openat+0xb6/0x306
         do_filp_open+0x3d/0x89
         open_exec+0x25/0xa0
         do_execve_common+0xea/0x2f9
         do_execve+0x43/0x45
         sys_execve+0x43/0x5a
         stub_execve+0x6c/0xc0

This is because prepare_bprm_creds grabs task->signal->cred_guard_mutex
and when do_lookup happens we try to grab task->signal->cred_guard_mutex
again in lock_trace.

Fix it using plain ptrace_may_access() helper in proc_map_files_lookup()
and in proc_map_files_readdir() instead of lock_trace(), the caller must
be CAP_SYS_ADMIN granted anyway.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Reported-by: Sasha Levin <levinsasha928@gmail.com>
Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Dave Jones <davej@redhat.com>
Cc: Vasiliy Kulikov <segoon@openwall.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -2177,16 +2177,16 @@ static struct dentry *proc_map_files_lookup(struct inode *dir,
 		goto out;
 
 	result = ERR_PTR(-EACCES);
-	if (lock_trace(task))
+	if (!ptrace_may_access(task, PTRACE_MODE_READ))
 		goto out_put_task;
 
 	result = ERR_PTR(-ENOENT);
 	if (dname_to_vma_addr(dentry, &vm_start, &vm_end))
-		goto out_unlock;
+		goto out_put_task;
 
 	mm = get_task_mm(task);
 	if (!mm)
-		goto out_unlock;
+		goto out_put_task;
 
 	down_read(&mm->mmap_sem);
 	vma = find_exact_vma(mm, vm_start, vm_end);
@@ -2198,8 +2198,6 @@ static struct dentry *proc_map_files_lookup(struct inode *dir,
 out_no_vma:
 	up_read(&mm->mmap_sem);
 	mmput(mm);
-out_unlock:
-	unlock_trace(task);
 out_put_task:
 	put_task_struct(task);
 out:
@@ -2233,7 +2231,7 @@ proc_map_files_readdir(struct file *filp, void *dirent, filldir_t filldir)
 		goto out;
 
 	ret = -EACCES;
-	if (lock_trace(task))
+	if (!ptrace_may_access(task, PTRACE_MODE_READ))
 		goto out_put_task;
 
 	ret = 0;
@@ -2241,12 +2239,12 @@ proc_map_files_readdir(struct file *filp, void *dirent, filldir_t filldir)
 	case 0:
 		ino = inode->i_ino;
 		if (filldir(dirent, ".", 1, 0, ino, DT_DIR) < 0)
-			goto out_unlock;
+			goto out_put_task;
 		filp->f_pos++;
 	case 1:
 		ino = parent_ino(dentry);
 		if (filldir(dirent, "..", 2, 1, ino, DT_DIR) < 0)
-			goto out_unlock;
+			goto out_put_task;
 		filp->f_pos++;
 	default:
 	{
@@ -2257,7 +2255,7 @@ proc_map_files_readdir(struct file *filp, void *dirent, filldir_t filldir)
 
 		mm = get_task_mm(task);
 		if (!mm)
-			goto out_unlock;
+			goto out_put_task;
 		down_read(&mm->mmap_sem);
 
 		nr_files = 0;
@@ -2287,7 +2285,7 @@ proc_map_files_readdir(struct file *filp, void *dirent, filldir_t filldir)
 					flex_array_free(fa);
 				up_read(&mm->mmap_sem);
 				mmput(mm);
-				goto out_unlock;
+				goto out_put_task;
 			}
 			for (i = 0, vma = mm->mmap, pos = 2; vma;
 					vma = vma->vm_next) {
@@ -2332,8 +2330,6 @@ proc_map_files_readdir(struct file *filp, void *dirent, filldir_t filldir)
 	}
 	}
 
-out_unlock:
-	unlock_trace(task);
 out_put_task:
 	put_task_struct(task);
 out: